The genus Concharium is the simplest and the most primitive form of all Concharida; it may be regarded as the common ancestral form of the whole family. The entire shell is usually almost spherical, without horns or teeth, and may be regarded as a Castanella which is bisected or broken into two equal hemispherical halves. The lateral margins of the two hemispherical valves are smooth, without teeth, and catch one into the other like the two valves of a Diatom, or the two halves of a bivalved box. Concharium agrees in this simple shape of the frontal margins with the following genus Conchasma, and represents with it the small subfamily Conchasmida.

1. Concharium bivalvum, n. sp. (Pl. 123, figs. 2, 2a).

Shell spherical, smooth. Diameter in all directions nearly the same. Borders of the two hemispherical valves circular, smooth, about twice as broad as the pores. In the half frontal perimeter of the shell (along the right and the left border of each valve) twenty-two to twenty-four pores, in the half sagittal perimeter (in the middle line of each valve) eighteen to twenty-two pores, in the half equator (in the cinctural perimeter of each valve) twenty to twenty-two pores. All pores circular, of the same size, twice as broad as their bars.

Dimensions.—Length of the shell (longitudinal diameter) 0.35, height (sagittal diameter) 0.34, breadth (lateral diameter) 0.33.

Habitat.—North Atlantic, west of Madeira, Station 354, depth 1675 fathoms.

2. Concharium nucula, n. sp. (Pl. 123, fig. 3).

Shell pear-shaped, with costate surface. Oral face somewhat truncated, broader than the aboral face. Its longitudinal diameter about one-fifth longer than the two other diameters. Borders of the two valves ovate, smooth, about as broad as the pores. In the half frontal perimeter of the shell twenty-two to twenty-four pores, in the half sagittal perimeter eighteen to twenty, in the half equator sixteen to eighteen. Pores irregularly roundish, three to four times as broad as the bars. The pores are so disposed in meridional rows that the crests between the rows converge towards the two poles of the sagittal axis.

Dimensions.—Length of the shell 0.2, height 0.18, breadth 0.16.

Habitat.—South Atlantic (west of Tristan da Cunha), Station 332, depth 2200 fathoms.

3. Concharium diatomeum, n. sp. (Pl. 123, fig. 1).

Shell nearly spherical, slightly lenticular, somewhat compressed in dorso-ventral direction; the sagittal diameter therefore somewhat shorter than the two others. Borders of the two hemispherical valves nearly circular, quite smooth, about as broad as the length of the largest pores. In the half frontal perimeter of the shell forty-four to fifty pores; in the half sagittal perimeter twenty to twenty-four; in the half equator thirty to thirty-three. Pores different in form and size; the marginal pores small, nearly circular; the dorsal and ventral pores oblongish-hexagonal, twice as long as broad, about four to six times as long as the bars, regularly arranged in transverse rows.

Dimensions.—Length of the shell 0.22, height 0.21, breadth 0.2.

Habitat.—Tropical Atlantic, near Sierra Leone, Station 348, depth 2450 fathoms.

4. Concharium bacillarium, n. sp. (Pl. 123, fig. 4).

Shell walnut-shaped, with panelled surface; oral and aboral face of the same form. Its longitudinal diameter about one-fifth longer than the two other diameters. Borders of the two cup-shaped valves elliptical, smooth, with a prominent edge, about as broad as the larger pores. In the half frontal perimeter of the shell fifty to fifty-five pores, in the half sagittal perimeter thirty-six to forty, in the half equator also thirty to forty. Pores hexagonally framed. The pores are tapering in size from the sagittal plane towards the valve-margins, and so regularly arranged in meridional rows that the crests between the latter converge towards both poles of the longitudinal axis.

Dimensions.—Length of the shell 0.2, height 0.15, breadth 0.15.

Habitat.—Tropical Atlantic, off St. Helena, Station 340, depth 1500 fathoms.

5. Concharium fragilissimum, n. sp.

Shell subspherical, very thin-walled and fragile. Diameter in all directions nearly the same. Oral and aboral face scarcely different. Margins of the hemispherical valves extremely thin and hyaline. Pores irregularly roundish, of very different sizes and unequal forms. The fragile shell of this species differs in general shape from that of all other Concharida, and is like that of the Cœlodendrida (Pl. 121, fig. 3), but exhibits neither an apical cupola or galea, nor radial tubes arising from it. It may be perhaps a young specimen of Cœlodendrum.

Dimensions.—Diameter of the shell 0.22, of the pores 0.002 to 0.02.

Habitat.—Mediterranean, Portofino (Haeckel), surface.

Genus 721. Conchasma,^[337] n. gen.

Definition.—Concharida with the lateral margins of the valves smooth, without sagittal keel, but with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchasma is closely allied to the preceding Concharium, and has the same hemispherical valves with smooth margins, without teeth; but it differs from the latter in the development of two caudal horns or posterior spines on the aboral hinge, one horn on the aboral end of each valve. The three species of this genus were all found in great depths of the Antarctic Ocean, in Diatom ooze, between 1260 and 1975 fathoms, at Stations 152 to 157.

1. Conchasma radiolites, n. sp. (Pl. 123, fig. 5).

Shell nearly spherical, somewhat compressed on both sides; the dorsal valve smaller, flatter and shorter than the ventral valve. In the half sagittal perimeter of the shell twenty to twenty-two pores, in the half frontal perimeter twelve to fourteen, in the half equator eighteen to twenty. All pores nearly of the same size, circular, hexagonally framed, scarcely as broad as the bars. The two horns of the hinge are four-sided pyramidal, of different sizes; the ventral horn (of the larger valve) two to three times as long as the dorsal horn (of the smaller valve); the latter twice as long as a pore.

Dimensions.—Length of the shell 0.16, height 0.15, breadth 0.14.

Habitat.—Antarctic Ocean, Station 154, depth 1800 fathoms.

2. Conchasma sphærulites, n. sp. (Pl. 123, fig. 6).

Shell nearly spherical, somewhat compressed on both sides, the frontal diameter therefore somewhat shorter than the two others. Both valves nearly of the same size, hemispherical, their borders smooth, twice as broad as the largest pores. In the half sagittal perimeter of the shell twenty-eight to thirty pores, in the half frontal perimeter twenty to twenty-two, in the half equator twenty-four to twenty-six. Size of the pores increasing from the borders towards the top of the valves. One series of very small pores along the frontal free margin of each valve. Pores roundish-polygonal, three to four times as broad as the bars. The two horns of the hinge are of equal size, four-sided pyramidal, and twice as long as the larger pores.

Dimensions.—Length of the shell 0.18, height 0.18, breadth 0.16.

Habitat.—Antarctic Ocean, Station 152, depth 1260 fathoms.

3. Conchasma hippurites, n. sp.

Shell nearly spherical, scarcely compressed. The frontal diameter equal to the two others. Both valves equal. In the half sagittal perimeter of the shell thirty-two to thirty-four pores, in the half frontal perimeter twenty-four to twenty-six, in the half equator twenty-six to twenty-eight. All pores of nearly equal size, circular, polygonally framed, twice as broad as the bars. The two horns of the hinge are large, three-sided pyramidal, the ventral horn twice as long as the dorsal, and four to six times as long as one pore.

Dimensions.—Length of the shell 0.22, height 0.21, breadth 0.2.

Habitat.—Antarctic Ocean, Station 157, depth 1950 fathoms.

Subfamily 2. Conchopsida, Haeckel.

Definition.—Concharida with the lateral margins of the two valves dentate, the teeth of both catch one into another.

Genus 722. Conchellium,^[338] n. gen.

Definition.—Concharida with the lateral margins of the valves dentate, without sagittal keel and without horns on the hinge.

The genus Conchellium and the four following genera represent together the subfamily Conchopsida, differing from the Conchasmida in the dentate lateral margins of the two valves. These are armed with a series of strong, conical teeth, and catch one into another just as the two valves of many Lamellibranchiata and Brachiopoda do (Pl. 124, figs. 1-16). Conchellium is the simplest form among the Conchopsida, since the valves are hemispherical, and possess neither a sagittal keel nor projecting horns.

1. Conchellium tridacna, n. sp. (Pl. 123, figs. 7, 7a).

Shell nearly spherical, finely tuberculated, the sagittal diameter somewhat longer than the two others. Borders of the two hemispherical valves smooth in 0.2 of the oral, and 0.1 at the aboral part, dentated in the remaining 0.7 part; on one side of each valve fourteen to sixteen very strong and long teeth, all nearly of the same size, about one-fourth as long as the shell-radius. In the half frontal perimeter of the shell (on one border of each valve) twenty-five to thirty pores, in the half sagittal perimeter thirty-five to forty, in the equator twenty-five to thirty. Pores circular, hexagonally framed, of equal size (except some smaller rows along the fissure), twice as broad as the bars. On the conical inside of each funnel-like pore six small spinules, between every three neighbouring pores a triangular facette (fig. 7a).

Dimensions.—Length of the shell 0.34 to 0.38, height 0.38 to 0.42, breadth 0.32 to 0.36.

Habitat.—North Pacific, Stations 250 to 253, depth 2740 to 3125 fathoms.

2. Conchellium hippopus, n. sp.

Shell nearly spherical, in the lateral perimeter (along the girdle-fissure) somewhat constricted. Borders of the two hemispherical valves semicircular, smooth in 0.3 of the oral, and 0.2 of the aboral part, dentated only in the remaining 0.5 middle part; on one side of each valve seven to eight very strong and long teeth, increasing in size towards the mouth, the longest (foremost) teeth nearly one-third as long as the shell-radius. In the half frontal perimeter of the shell (along one border of each valve) twenty-four to twenty-eight pores, in the half sagittal perimeter thirty-two to thirty-six, in the half equator twelve to fourteen. Pores circular, twice to three times as broad as the bars, smaller along the fissure.

Dimensions.—Length of the shell 0.06, height 0.065, breadth 0.055.

Habitat.—Central area of the Tropical Pacific, Station 274, depth 2750 fathoms.

Genus 723. Conchidium,^[339] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

Definition.—Concharida with the lateral margins of the valves dentate, without sagittal keel and apical horns, but with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchidium is the most common form of all Concharida, and some of its species occur in great numbers in the tropical zone of the Pacific and the Atlantic, on the surface as well as at various depths. It differs from the preceding Conchellium, its ancestral form, in the development of two caudal horns, or two strong pyramidal spines which arise from the posterior end of the valves; the dorsal horn usually is smaller than the ventral.

1. Conchidium terebratula, n. sp. (Pl. 124, figs. 1, 2).

Shell subspherical, smooth; both valves of nearly equal size and form, hemispherical. The three dimensive axes of the body are almost equal. Margins of the valves dentate in nearly the whole periphery; on each side of one valve eleven or twelve strong conical teeth, all of the same size. Aboral hinge with two short and stout four-sided pyramidal horns of equal length. Mouth with two equal short lips. Pores of the shell subregular, circular, three to four times as broad as the bars, in the dorsal valve twice as large as in the ventral valve.

Dimensions.—Diameter of the shell 0.24 to 0.28.

Habitat.—Central Pacific, Stations 270 to 274, surface, and at various depths.

2. Conchidium thecidium, n. sp. (Pl. 124, fig. 6).

Shell subspherical, slightly compressed on both sides. Dorsal valve somewhat smaller than the ventral, of similar form. Principal axis of the shell somewhat longer than the sagittal, and this longer than the frontal axis. Margins of the valves smooth in the oral quarter, strongly dentate in the remainder; on each side of one valve eight or nine very large triangular teeth, half as long as the height of the valve. Aboral hinge with two unequal, stout, four-sided pyramidal horns; the dorsal horn half as long at the ventral. Mouth with two unequal lips, the upper shorter than the lower. Pores of the shell subregular, hexagonal, three to four times as broad as the bars. The figured specimen, well preserved, contained in the central capsule two nuclei, one in the dorsal, the other in the ventral half.

Dimensions.—Length of the shell 0.28 to 0.33, height 0.27 to 0.3, breadth 0.22 to 0.24.

Habitat.—South Pacific, Station 292, depth 1600 fathoms.

3. Conchidium rhynchonella, n. sp. (Pl. 124, fig. 3).

Shell with three different dimensive axes in the proportion = 6 : 5 : 4. Ventral valve semi-ovate, larger than the humpbacked dorsal valve. Margins of the valves dentate almost in the whole periphery; on each side of one valve twelve or thirteen strong conical teeth. Aboral hinge with two very unequal horns, the dorsal much smaller than the pyramidal ventral. Under lip of the mouth emarginate, much larger than the acute upper lip. Pores subregular, circular, twice as broad as the bars.

Dimensions.—Length of the shell 0.3, height 0.25, breadth 0.2.

Habitat.—North Pacific, Stations 244 to 253, surface, and at various depths.

4. Conchidium dimerella, n. sp.

Shell laterally compressed, very similar to the preceding, but differing in the following characters:—Proportion of the three axes = 7 : 5 : 4. Ventral valve hemispherical, larger than the humpbacked dorsal. Margins of the valves dentate in the middle half only, whilst the anterior and posterior quarters are smooth; on each side of one valve six or seven strong conical teeth. Ventral horn of the aboral hinge very large, pyramidal, one-third as long as the shell and three times as long as the dorsal.

Dimensions.—Length of the shell 0.35, height 0.25, breadth 0.2.

Habitat.—North Pacific, Station 236, surface.

5. Conchidium leptæna, n. sp. (Pl. 124, figs. 4, 5).

Shell laterally compressed, with three different dimensive axes of the proportions = 4 : 3 : 2. Ventral valve larger than the dorsal, both semi-ovate. Margins of the valves dentate along the lateral sides; on each side of one valve nine or ten conical teeth, the middle of which are smaller. Aboral hinge with two unequal short horns; the dorsal rudimentary. Under lip of the mouth pointed, much larger than the truncate upper lip. Pores circular, about twice as broad as the bars.

Dimensions.—Length of the shell 0.2, height 0.15, breadth 0.1.

Habitat.—Central Pacific, Stations 271 and 272, surface, and at various depths.

6. Conchidium argiope, n. sp. (Pl. 124, figs. 7-9).

Shell laterally compressed, with three different dimensive axes of the proportion = 6 : 5 : 3. Both valves nearly equal. Margins of the shell dentate along the lateral fissure, on each side of one valve thirteen or fourteen thin conical teeth. Aboral hinge with two equal, short, pyramidal horns. Mouth with two equal short lips. Pores subregular, circular, twice as broad as the bars.

Dimensions.—Length of the shell 0.2, height 0.17, breadth 0.1.

Habitat.—Tropical Atlantic, Stations 341 to 349, surface, and at various depths.

7. Conchidium magasella, n. sp.

Shell laterally compressed, with two equal valves, very similar to the preceding species, but differing in the following characters:—Proportion of the three axes = 6 : 4 : 3. Margins of the valves with smaller and more numerous teeth; on each side of one valve sixteen to eighteen short conical teeth. Pores smaller and more numerous, about as broad as the bars.

Dimensions.—Length of the shell 0.24, height 0.16, breadth 0.12.

Habitat.—Indian Ocean, Madagascar (Rabbe), surface.

8. Conchidium productum, n. sp.

Shell laterally compressed, prolonged, with two equal valves, similar to the two preceding species, differing in the following characters:—Proportion of the three axes = 3 : 2 : 1. Margins of the valves smooth in the anterior and posterior quarter, dentate in the middle lateral half; on each side of one valve ten to twelve strong conical teeth. Horns of the aboral hinge prolonged, conical, half as long as the shell, the ventral somewhat larger than the dorsal. Pores regular, circular, twice as broad as the bars.

Dimensions.—Length of the shell 0.25, height 0.15, breadth 0.08.

Habitat.—North Atlantic, Station 354, surface.

Genus 724. Conchonia,^[340] n. gen.

Definition.—Concharida with the lateral margins of the valves dentate, without sagittal keel, but with an apical horn on the poles of the sagittal axis, and with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchonia is closely allied to the preceding Conchidium, its ancestral form, but differs from this and from all other Concharida in the development of horns on the poles of the sagittal axis. These are probably of great morphological importance, since they represent the beginnings of the hollow tubes arising from the poles of the sagittal axis in all Cœlodendrida and Cœlographida. In one of the three observed species each valve possessed an apical or sagittal horn, whilst in the two other species one valve only was provided with a horn. Since I observed one specimen only of each species, I cannot say whether this difference is important and of constant generic value.

1. Conchonia diodon, n. sp. (Pl. 124, figs. 10-12).

Shell laterally compressed, with two very unequal valves. Dorsal valve smaller, hat-shaped, on the apex with a fenestrated protuberance which is similar to the galea of the Cœlodendrida, and bears a short, conical, backwardly-directed horn. Ventral valve larger, boat-shaped, without apical horn. Aboral hinge with two pyramidal, horizontal, caudal horns of different sizes, the dorsal smaller than the ventral. Lateral margin of each valve on one side with twelve to fourteen strong conical teeth (fig. 12). Lips of the narrow mouth thickened (fig. 11).

Dimensions.—Length of the shell 0.3, height 0.27, breadth 0.21.

Habitat.—Tropical Atlantic, Station 342, depth 1445 fathoms.

2. Conchonia triodon, n. sp. (Pl. 124, figs. 13, 14).

Shell laterally compressed, with two very unequal valves. Dorsal valve (fig. 14) larger, hat-shaped, on the apex with a large pyramidal horn which is half as long as the shell, curved and directed backwards. Ventral valve (fig. 13) smaller, boat-shaped, without apical horn. Aboral hinge with two pyramidal caudal horns of different sizes, the dorsal horn twice as long as the ventral. Lateral margin of each valve on one side with twelve to fifteen conical teeth. Perhaps the larger horned valve (fig. 14) may be the ventral, and the opposite smaller hornless (seen from above in fig. 13) the dorsal valve.

Dimensions.—Length of the shell 0.21, height 0.17, breadth 0.12.

Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

3. Conchonia tetrodon, n. sp.

Shell subspherical, with two nearly equal hemispherical valves, which are very thin-walled and similar in structure to those of the Cœlodendrida, with very irregular roundish pores of different shapes and sizes (compare Pl. 121, fig. 3). Lateral margins of the valves with very numerous and irregular, thin, bristle-shaped teeth, similar to those of some Cœlographida (compare Pl. 127, fig. 8). Aboral hinge with two equal, conical, caudal horns, which are straight, parallel, and half as long as the shell. Two similar straight conical horns are opposed on the poles of the sagittal axis, and arise from the apex of the two valves. This remarkable species may perhaps better represent a separate genus, Conchura, forming a direct transition to the ancestral form of the Cœlodendrida, Cœlodoras; it differs from the latter in the absence of a galea or hollow conical cupola on the apex of each valve, and in the solid, not hollow structure of the horns.

Dimensions.—Diameter of the shell 0.24, length of the two sagittal horns 0.1, of the two caudal horns 0.12.

Habitat.—Indian Ocean, Cocos Islands (Rabbe), surface.

Genus 725. Conchopsis,^[341] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

Definition.—Concharida with dentate lateral margins and a sharp sagittal keel of the compressed valves, without horns on the hinge.

The genus Conchopsis and the following Conchoceras differ from the other Concharida in the strong lateral compression of the shell, so that each valve is provided in the sagittal plane with a sharp prominent keel, comparable to the dorsal and the anal fin of fishes. These compressed shells are in general twice to three times as large as the more roundish and keelless shells of the five preceding genera. The sculpture of the fenestrated valves is extremely elegant. Conchopsis possesses at the aboral hinge not the two prominent caudal horns, which mark the following genus Conchoceras, but in some species a peculiar ligament connects the aboral ends of both valves.

1. Conchopsis orbicularis, n. sp. (Pl. 125, fig. 3).

Shell subcircular, lenticular, strongly compressed on both sides, nearly as high as long, its sagittal perimeter nearly circular; frontal and cinctural perimeter spindle-shaped. Borders of the two boat-shaped valves smooth in 0.4 of the oral part, and in 0.1 of the aboral part of their length, strongly dentated in the remaining 0.5 middle part; about twenty-five slender, straight teeth on each side of one valve, size of the teeth increasing from the aboral towards the oral pole. In the half lateral perimeter of the shell (along the right and the left borders of each valve) sixty to sixty-five pores, in the half sagittal perimeter (along the keel of each valve) eighty to eighty-five pores, in the half equator sixty to sixty-five pores.

Dimensions.—Length of the shell 0.53, height 0.55, breadth about 0.2.

Habitat.—South Atlantic, west of Tristan da Cunha, Station 333, depth 2025 fathoms.

2. Conchopsis compressa, n. sp. (Pl. 125, figs. 7, 8).

Shell lenticular, strongly compressed on both sides; proportion of its longitudinal diameter to the sagittal and lateral = 10 : 9 : 3, its sagittal perimeter elliptical (fig. 7), cinctural and frontal perimeter spindle-shaped (fig. 8). Borders of the two boat-shaped valves smooth in 0.3 of the oral, and 0.3 in the aboral part, dentated in the remaining 0.4 middle part; about forty to forty-four very slender teeth of equal size on one lateral edge of each valve. In the half frontal perimeter of the shell (along the border of the valve) sixty-four to sixty-eight pores, in the half sagittal perimeter (along one valve-keel) seventy to eighty pores, in the half equator forty-four to forty-eight pores. Ventral and dorsal pores linear, three to four times as long as the circular, lateral pores.

Dimensions.—Length of the shell 0.6 to 0.8, height 0.55 to 0.72, breadth 0.2 to 0.3.

Habitat.—North Pacific, between 30° and 40° north latitude (between Japan and San Francisco), in depths from 2000 to 3000 fathoms frequent, Stations 241 to 252.

3. Conchopsis carinata, n. sp. (Pl. 123, fig. 8).

Shell subcircular, lenticular, in the central half slightly compressed, nearly spherical, in the peripheral half strongly compressed, with a broad, hyaline, smooth keel on the sagittal plane. Borders of the two valves smooth in the 0.2 of the oral, and 0.1 of the aboral part, strongly dentated in the remaining 0.7 middle part; about fifty slender teeth of equal size on one lateral edge of each valve. In the half lateral perimeter of the shell about forty-five to fifty pores, in the half sagittal perimeter sixty-five to seventy, in the half equator thirty-two to forty pores. The pores are arranged in parallel curved rows, which are separated by high denticulate crests. Aboral hinge with a strong ligament.

Dimensions.—Length of the shell 0.6 to 0.7, height 0.55 to 0.65, breadth 0.35 to 0.45.

Habitat.—South Atlantic (east of Patagonia), Station 318, depth 2040 fathoms.

4. Conchopsis lenticula, n. sp. (Pl. 123, fig. 9).

Shell subcircular, lenticular, strongly compressed on both sides, with a sharp and broad hyaline keel in the sagittal perimeter. Borders of the two valves smooth in 0.3 of the oral, and 0.2 of the aboral part, strongly dentated in the remaining 0.5 middle part; about thirty strong, conical teeth on the lateral edge of each valve. In the half lateral perimeter of the shell fifty to fifty-five pores, in the half sagittal seventy to seventy-five, in the half frontal forty to forty-five pores. The large central capsule of this species fills up the posterior half of the shell-cavity, the dark green phæodium the anterior half; the latter contains numerous peculiar, longish, nucleated cells (fig. 9a), parasites or symbiontes (?). Aboral hinge of the shell with a strong ligament.

Dimensions.—Length of the shell 0.7, height 0.6, breadth 0.3.

Habitat.—Central Pacific, Stations 271 to 274, depth 2350 to 2750 fathoms.

5. Conchopsis pilidium, n. sp. (Pl. 125, fig. 9).

Shell ovate, lenticular, compressed on both sides, with a broad, wing-like sagittal keel. Proportion of its longitudinal diameter to the sagittal and lateral = 6 : 5 : 3. Sagittal perimeter elliptical. Borders of the two hat-like valves smooth in 0.15 of the oral, and 0.2 of the aboral part, strongly dentated in the remaining 0.65 middle part; about twenty-five to thirty teeth of nearly equal size on one side of each valve. In the half lateral perimeter of the shell (along one border of each valve) forty-five to fifty pores, in the half sagittal perimeter (on the keel of one valve) seventy to seventy-five pores, in the half equator thirty-six to forty pores. Each pore is surrounded by a hexagonal frame. The opening of each valve in this species is bordered and partly closed by a broad, horizontal diaphragm or velum, like the deck of a boat; it is broadest on the oral side.

Dimensions.—Length of the shell 0.78 to 0.8, height 0.66 to 0.7, breadth 0.3 to 0.4.

Habitat.—South Atlantic, between Buenos Ayres and Tristan da Cunha, Stations 324 to 334, at depths between 1715 and 2900 fathoms.

6. Conchopsis aspidium, n. sp. (Pl. 125, figs. 1, 2).

Shell scutiform, strongly compressed on both sides, in the centre only lenticular, in the periphery wing-like, keeled. Proportion of the longitudinal diameter to the sagittal and frontal = 6 : 5 : 2. Sagittal circumference in the oral half semicircular, in the aboral half pentagonal, two acute corners jutting out near the aboral hinge, one corner in the keel of the dorsal, the outer in the keel of the ventral valve. Borders of the two boat-shaped valves smooth in 0.3 of the oral part, and in 0.1 of the aboral part, strongly dentated in the remaining 0.6 middle part; about thirty-five teeth in one side of each valve, larger on both ends than in the middle. In the half frontal perimeter of the shell sixty to sixty-five pores, in the half sagittal perimeter eighty to eighty-five, in the half equator of the shell forty to fifty pores.

Dimensions.—Length of the shell 0.55 to 0.65, height 0.5 to 0.55, breadth 0.2 to 0.22.

Habitat.—North Pacific, Stations 243 and 244, depth 2800 to 2900 fathoms.

7. Conchopsis navicula, n. sp. (Pl. 125, figs. 4-6).

Shell pear-shaped, compressed on both sides, in the sagittal periphery keeled. Proportion of the longitudinal diameter to the sagittal and lateral = 4 : 3 : 2. Its sagittal perimeter nearly ovate. Borders of the two boat-shaped valves smooth in 0.3 of the oral, and 0.15 of the aboral part, strongly dentated in the remaining 0.55 middle part; teeth conical, of nearly equal size. In the half frontal perimeter of the shell (along one border of each valve) forty to forty-five pores, in the half sagittal fifty-four to fifty-six pores, in the half equator thirty-two to thirty-six pores. Each pore is surrounded by a hexagonal frame, and pierces the shell in an oblique direction, dilated in the middle part (figs. 5, 6). Shell very thick-walled, several longitudinal crests on both sides of the keel of each valve. Hinge very strong, usually with a broad ligament between the two unequal aboral lips of the hinge.

Dimensions.—Length of the shell 0.8, height 0.6, breadth 0.4.

Habitat.—South Pacific, Station 293, depth 2025 fathoms.

Genus 726. Conchoceras,^[342] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

Definition.—Concharida with dentate lateral margins and a sharp sagittal keel of the compressed valves, and with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchoceras has the same lenticular keeled and laterally compressed shell as the preceding ancestral genus Conchopsis, but is distinguished from it by the development of two large caudal horns on the aboral hinge. It bears therefore the same relation to the latter as Conchidium does to Conchellium.

1. Conchoceras caudatum, n. sp. (Pl. 124, fig. 15).

Shell lenticular, slightly compressed; proportion of the longitudinal diameter to the sagittal and lateral = 6 : 5 : 4; sagittal and cinctural perimeter ovate, frontal perimeter elliptical. Free margins of the two boat-shaped valves dentate nearly in the whole perimeter; in one lateral border of each valve eleven or twelve very large conical teeth, the largest of which are nearly half as high as the valve. On the aboral hinge of the shell two divergent, very large horns, four-sided pyramidal, acute, straight, at the base perforated by a few large pores. The dorsal horn (of the smaller valve) is somewhat shorter than the ventral horn (of the larger valve). The apical distance of both horns is somewhat greater than their length, and about half the length of the shell. In the half frontal perimeter (along one border of each valve) thirty-two to thirty-four pores, in the half sagittal perimeter thirty-four to thirty-eight, in the half equator thirty to thirty-three. The pores are smaller near the girdle-fissure, irregularly quadrangular, and arranged in longitudinal rows, which are separated by meridional crests, and converge towards both poles of the main axis.

Dimensions.—Length of the shell (without the horns) 0.24 to 0.26, height 0.20 to 0.22, breadth 0.16 to 0.18; length of the horns 0.1 to 0.13.

Habitat.—Eastern part of the Tropical Atlantic, near the Equator, Station 348, depth 2450 fathoms.

2. Conchoceras cornutum, n. sp. (Pl. 124, fig. 16).

Shell lenticular, strongly compressed on both sides; proportion of the longitudinal axis to the sagittal and lateral = 4 : 3 : 2. Sagittal perimeter ovate, frontal perimeter elliptical, cinctural perimeter spindle-shaped. Free margins of the two keeled valves in the oral third smooth, in the remaining part strongly dentate; ten or eleven large teeth on one lateral border of each valve, the largest about half as high as the valve. On the aboral hinge of the shell two fenestrated apophyses which bear two stout, strongly curved horns, like pincers; the dorsal horn (of the smaller upper valve) is shorter and less curved than the ventral horn (of the larger lower valve). The lips of the mouth (at left on fig. 16) are also unequal, the upper lip emarginate. In the half frontal perimeter of the valve twenty-two to twenty-four pores, in the half sagittal perimeter twenty-eight to thirty, in the half equator twenty to twenty-two. The pores are separated by high parallel crests and arranged in longitudinal rows, which converge towards the aboral hinge.

Dimensions.—Length of the shell (without the horns) 0.36 to 0.4, height 0.27 to 0.3, breadth 0.2 to 0.22; length of the horns 0.16 to 0.22.

Habitat.—Tropical Atlantic, Station 338, depth 1990 fathoms.

Family LXXXIV. Cœlodendrida, Haeckel (Pl. 121).

Definition.—Phæodaria with a bivalved lattice-shell, composed of two hemispherical valves, a dorsal and a ventral. A conical cupola or a pyramidal galea arises from the apical pole of both valves, therefore at the opposite poles of the sagittal axis. Rhinocanna and frenula wanting. Three or more hollow radial tubes arise from each valve and are symmetrically disposed. Sometimes their branches form an outer bivalved mantle. The central capsule is so enclosed between the two inner valves, that its three openings lie in the open frontal fissure between them.

The family Cœlodendrida differs from the preceding Concharida (its probable ancestral group) in the development of a conical galea or pyramidal cupola on the apical pole of each valve, and of three or more hollow radial tubes arising from each galea. They do not possess, however, the peculiar sagittal nasal tube or rhinocanna, which is constantly developed from the base of each cupola (and connected with its apex by a frenulum) in the following family, the Cœlographida. These latter differ also from the former in the constant possession of prominent verticillate styles.

The family Cœlodendrida was founded in 1862 in my Monograph (p. 360) and represented hitherto only by two species of the genus Cœlodendrum, there described (p. 361, Taf. xiii. figs. 1-3, and Taf. xxxii. fig. 1). This first description, however, contained some errors, which were afterwards (in 1879) corrected by Richard Hertwig; this author also gave the first accurate description of the central capsule and its three openings. In the rich collection of the Challenger, the Cœlodendrida are represented by four genera, but only seventeen species, some of which, however, are cosmopolitan and very common, particularly Cœlodendrum.

The two valves of the lattice-shell, dorsal and ventral, are either hemispherical, or somewhat more flatly vaulted or cap-shaped. They are never connected in the equatorial zone of the body, as I supposed in my Monograph (1862, loc. cit.); but they are separated by the girdle-fissure, a free circular equatorial interval, in which lie the three openings of the enclosed central capsule. Though the two valves, therefore, have no direct connection, they are, however, always opposed so accurately, that their equal free circular edges correspond exactly one to the other, so that the apex of each valve lies in one pole of the sagittal axis. From this apex there arises on each valve an irregular conical or three-sided pyramidal cupola, the galea (Pl. 121, figs. 3, 4, 8). The Cœlodendrida differ in the possession of this galea from the Concharida, and agree with the Cœlographida; but they never exhibit the peculiar rhinocanna or nasal tube, which arises from each galea in the latter family.

The siliceous lattice-plate of the two valves, and of the galea arising from them, is very thin and fragile, and its irregular roundish pores are extremely variable in size, number, and disposition. Sometimes the pores are so small and so scarce, that the plate appears nearly solid. At other times the siliceous plate seems to be really solid, and covered by a network of thin crests, the small dimples between which give to it the appearance of being fenestrated. Often the pores or the dimples are wanting in the central part of each valve, while they are very numerous and dense in the peripheral part. The same may be said of the lattice-plate of the galea, which is sometimes nearly solid, at other times richly fenestrated. The Cœlodendrida agree in this structure with the following family, the Cœlographida, and differ from the preceding family, the Concharida, in which the siliceous wall of the two valves is much thicker, and perforated by regular circular or roundish pores.

The galea or conical cupola in the apex of the two valves ("der kegelförmige Aufsatz" of the German authors) has in all Cœlodendrida a triangular base and an irregularly conical or nearly three-sided pyramidal form. Its cavity is about one-third or one-fourth as large in diameter as the cavity of the hemispherical valve upon which it rests. The galea is relatively smaller and more irregularly formed than in the Cœlographida, and differs essentially from that of the latter in the constant absence of a rhinocanna; there are also wanting, therefore, the characteristic frenula, which connect the nasal tube with the apex of the galea. The cavity of the galea probably always communicates with that of the valves by pores in the separating siliceous plate, and is besides pierced by irregular pores in its outer wall, very variable in form, size, and number, but it does not communicate with the cavity of the hollow radial tubes, from which it is separated by a thin, solid, siliceous plate.

The hollow radial tubes which arise from the galea in the Cœlodendrida do not seem to possess that constant regularity in number, origin, and disposition, which is found in the following family, and there serves for distinction of genera. In my first description of the Cœlodendrida (1862, loc. cit., p. 362), I pointed out this irregularity, and mentioned that the number of radial tubes arising from each galea varies from three to eight; the total number therefore amounts to from six to sixteen, the same minimum and maximum numbers which we shall encounter also in the radial styles of the following family. But whilst it is easy to determine the position and relation of these hollow tubes in the Cœlographida, owing to the constant sagittal position of their rhinocanna, this task is very difficult in the Cœlodendrida, where the rhinocanna is wanting. In the most frequent cases there arise from each galea three or four tubes, more rarely five or six, and very rarely seven or eight. The simplest and probably the original case is the development of three tubes, two of which are paired (divergent on the right and left), while the third is odd, lying in the sagittal plane. Perhaps these three primary tubes may be compared to the three cortinar feet of the Nassellaria, so that we may regard the two paired anterior as pectoral, and the odd posterior as a caudal tube. Usually the two paired or pectoral tubes arise from two corners of the triangular base of the galea, whilst the third odd or caudal tube does not arise from the third corner of the base, but more or less above it, and often even from the highest point or the apex of the galea. In the majority of species observed, this odd sagittal tube is forked even at its origin, so that two divergent tubes (an anterior and a posterior) arise from the apex of the galea (Pl. 121, figs. 3, 8). More rarely the two paired or pectoral tubes are also forked at the base, so that three pairs of tubes arise from each galea, and the total number of tubes amounts to twelve. Very rarely four separate tubes or four pairs of tubes arise from each galea, viz., two from the two anterior corners of the basal triangle, one from the posterior corner, and one from the apex of the galea. It is possible that this difference in the origin, furcation, and number of the hollow radial tubes may be employed for the distinction of genera of Cœlodendrida, in the same manner as it is employed in the next following family, the Cœlographida. But I have not been able, in spite of numerous and accurate examinations, to demonstrate in the former the same regularity in number and arrangement of the tubes as in the latter. It seems that these relations here are very variable, even in one and the same Species, and not yet fixed.

It is, however, probable, on the other hand, that the primary tubes (all or partly) are identical to the Cœlodendrida and Cœlographida. This is most probably the case with the posterior odd or caudal tube, which seems to be never wanting, and in both families is developed in the form of a dichotomous brush (never in the form of a verticillate style). Possibly also the two paired pectoral tubes are homologous in both families.

The hollow tubes are perfectly simple and unbranched only in one genus, Cœlodoras, which is probably the common ancestral form of both families, and which may have been derived from Concharium by development of a galea and tubes on the sagittal apex of the valves. All the other Cœlodendrida have branched spines, and the ramification is constantly dichotomous, or repeatedly forked. There never occur in this family those characteristic "styles," or verticillate prolonged tubes, which we find in all Cœlographida. Usually the cylindrical tubes are slightly curved and forked even near their base. The furcation is repeated a variable number of times in the different species. In the largest species each tube becomes a brush with more than one hundred terminal bristles.

We divide the Cœlodendrida into two subfamilies, according to the different development of the distal branches of the hollow tubes. In the Cœlodorida all the branches of the tubes remain free and are never connected by anastomoses, so that the surface of the bivalved skeleton is protected by the free radial distal branches of the tubes. In the larger species of Cœlodendrum (e.g., Cœlodendrum furcatissimum, Pl. 121, fig. 1), the numerous branches of the dichotomous tubes form a dense thicket, similar to that in the Cœlotholida.

In the second subfamily, Cœlodrymida, the distal branches of the tubes are connected by numerous anastomoses, and compose either a simple lattice-plate on the surface of the skeleton (Cœlodrymus), or a thicker envelope of spongy framework (Cœlodasea). The lattice-mantle so produced is always bivalved, and its two outer hemispherical valves (dorsal and ventral) correspond exactly to the two inner valves, from which arise the hollow tubes. The free margins of the two external mantle-valves come externally into contact in the equatorial plane of the body, in which the girdle-fissure lies internally between the two central shell-valves. The free edges of the two external mantle-valves, opposed to one another in the circle of the equator, seem usually to catch one into another in the same way as the corresponding mantle-valves of the Cœloplegmida are loosely connected (Pl. 128, figs. 1, 7). A true concrescence between the two valves seems never to take place.

The two subfamilies of Cœlodendrida therefore exactly correspond to the two subfamilies of the following family, the Cœlographida. The Cœlodorida and Cœlotholida form in a similar way a thicket, by dichotomous ramification of the hollow tubes, all the branches of which remain free. The Cœlodrymida and Cœloplegmida, on the other hand, form an outer lattice-mantle by anastomosing branches. The latter two subfamilies, of course, have been derived correspondingly from the two former, and the common ancestral form of all four is probably Cœlodoras, derived from the Concharida.

Though the two corresponding subfamilies in both groups are very similar, they are, however, separated by important hereditary characters. All Cœlodendrida (the Cœlodorida without a mantle as well as the Cœlodrymida with a mantle) possess no rhinocanna and no frenula on the galea, and they never develop prominent verticillate styles; the surface of their calymma is probably always spherical or subspherical. All Cœlographida, however (the Cœlotholida without a mantle as well as the Cœloplegmida with a mantle), possess a rhinocanna and frenula on the galea, and always develop prominent verticillate styles; the surface of their calymma is probably always symmetrically polyhedral.

The superficial armature of the skeleton in the Cœlodendrida is rather simple, and by no means so manifold and differentiated as in the more highly developed Cœlographida. The thin terminal branches of the hollow tubes are in the Cœlodorida closed at the distal end, and armed with a variable number of short teeth (Pl. 121, fig. 2), or with a spinulate terminal knob, or a corona of recurved hooks (ibid., figs. 5-7). In the Cœlodrymida, however, where the distal ends of the branches by anastomosing form the lattice-mantle, the spherical surface of this latter is armed with numerous thin spathillæ or radial bristles (often zig-zag or spinulate), and each bristle usually bears at the distal end a small anchor with two, three, or four recurved teeth; the outer convex edge of these teeth is usually smooth, the inner concave edge denticulate. All these ramules and branches of the tubes (also the thinnest terminal threads) are hollow, and filled up by jelly.

The central capsule of the Cœlodendrida does not lie outside the two central valves (as I supposed in my first description, in 1862, being deceived by the dark enveloping phæodium, Monogr. d. Radiol., Taf. xxxii. fig. 1), but it is enclosed between the two valves, as in the preceding and the following family. The first accurate description of it was given by Richard Hertwig in 1879 (loc. cit., p. 95, Taf. x. fig. 3). Its constant position between the two lattice-valves (dorsal and ventral) is such, that its three openings lie in the frontal plane, in the open fissure between the valves. The astropyle or the main-opening, with the radiate operculum and the tubular proboscis arising from it, lies on the anterior (or oral) pole of the main axis, whilst the two lateral accessory openings, or parapylæ, lie on both sides of the posterior (or aboral) pole, to the right and left. The position of the capsule is therefore the same as in the preceding Concharida (Pls. 123-125), and the following Cœlographida (Pls. 126-128). The large nucleus, enclosed in the central capsule, is usually half as broad, and contains numerous nucleoli.

The calymma, or the extracapsular jelly-veil, is in the Cœlodendrida usually spherical, very voluminous, and includes the entire skeleton, the thicket of the Cœlodorida as well as the lattice-mantle of the Cœlodrymida. Only the outermost terminal branches of the tubes in the former, and the radial bristles and spathillæ on the surface of the latter, remain free and project beyond the surface of the calymma. The phæodium is usually very large, three to four times as broad as the central capsule, and envelops it often completely. Usually it envelops only the anterior half of it, and the proboscis (Pl. 121, figs. 1, 9). Often numerous green, brown, or blackish phæodellæ are scattered through the whole calymma, and sometimes accumulate in a superficial layer on its surface. The galea of both valves is usually also filled up by the phæodium.

Synopsis of the Genera of Cœlodendrida.

Subfamily I. Cœlodorida, Haeckel.

Definition.—Cœlodendrida without an external bivalved lattice-mantle, with simple or branched hollow tubes, the terminal branches of which are free, not anastomosing.

Genus 727. Cœlodoras,^[343] n. gen.

Definition.—Cœlodendrida without external lattice-mantle, with simple, not branched, radial tubes, which arise separately from the galea.