Shell subovate, with two distinct strictures. Length of the three joints = 2 : 4 : 3, breadth = 3 : 5 : 4. Cephalis hemispherical, with a short conical spine of the same length. Thorax subspherical, with three broad triangular wings of half the length arising from its lower half. Abdomen subconical. Pores small, regular, circular, of nearly equal size.

Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.03; breadth, a 0.03, b 0.05, c 0.04.

Habitat.—Fossil in Barbados.

5. Lithornithium fringilla, n. sp. (Pl. 67, fig. 2).

Shell ovate, with two slight strictures. Length of the three joints = 4 : 7 : 4, breadth = 3 : 7 : 5. Cephalis ovate, with a pyramidal spine of the same length Thorax subconical, with three stout conical, curved wings of half the length, arising from its lower third (immediately above the girdle). Abdomen inversely campanulate. Pores subregular, circular, of equal size.

Dimensions.—Length of the three joints, a 0.04, b 0.07, c 0.04; breadth, a 0.03, b 0.07, c 0.05.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

6. Lithornithium hirundo, Ehrenberg.

Shell nearly ovate, with two deep strictures. Length of the three joints = 2 : 6 : 4, breadth = 2 : 7 : 5. Cephalis hemispherical, with an oblique, conical horn of twice the length. Thorax hemispherical, with three long, angular, little divergent wings of twice the length, which are S-shaped, curved, and arise from its lower third, immediately above the girdle. Abdomen inversely conical. Pores irregular, roundish, of different sizes. In the figure of Ehrenberg the abdomen is broken off; in a specimen from Caltanisetta, with somewhat shorter and broader wings, I found it complete.

Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.04; breadth, a 0.02, b 0.07, c 0.05.

Habitat.—Fossil in Tertiary rocks of the Mediterranean (Ægina, Greece; Caltanisetta, Sicily).

Genus 596. Sethornithium,^[213] Haeckel, 1881, Prodromus, p. 436.

Definition.—Theoperida (vel Tricyrtida triradiata clausa) with three latticed lateral wings on the thorax.

The genus Sethornithium differs from the preceding Lithornithium, its ancestral form, only in the fenestration of the three thoracic wings, and bears therefore to it the same relation that Dictyoceras exhibits to Pterocorys.

1. Sethornithium dictyopterum, n. sp. Pl. 68, fig. 14.

Shell ovate, with two slight strictures. Length of the three joints = 2 : 4 : 3, breadth = 1 : 5 : 4. Cephalis ovate, with a pyramidal horn of twice the length. From the middle part of the thorax arise three broad, triangular, latticed wings of about the same length, the distal end of each of which is curved downwards. Abdomen inversely conical. Pores regular, circular. (Similar to Lithornithium fringilla, Pl. 67, fig. 2, but with much larger wings, which in the greater part are fenestrated.)

Dimensions.—Length of the three joints, a 0.04, b 0.08, c 0.06; breadth, a 0.02, b 0.1, c 0.08.

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

Genus 597. Theopera,^[214] Haeckel, 1881, Prodromus, p. 436.

Definition.—Theoperida (vel Tricyrtida triradiata clausa) with three lateral wings, beginning from the sides of the thorax and prolonged into the sides of the abdomen.

The genus Theopera differs from the two preceding genera in the greater extension of the three lateral wings, which are prolonged from their original base, the thorax, into the inversely conical abdomen. It has therefore an intermediate position between the preceding and the following genera. It differs from the similar Pterocanium and Theopodium in the closure of the constricted and fenestrated mouth.

1. Theopera prismatica, n. sp. (Pl. 67, fig. 7).

Shell three-sided prismatic, with three broad and long hyaline wings, which are prolonged from the collar stricture almost to the basal end, with three parallel edges. Length of the three joints = 2 : 8 : 10, breadth = 3 : 9 : 8. Cephalis hemispherical, with a conical horn of the same length. Thorax subspherical; abdomen inversely ovate; pores of both small, regular, circular, of equal size.

Dimensions.—Length of the three joints, a 0.02, b 0.08, c 0.1; breadth, a 0.03, b 0.09, c 0.08.

Habitat.—South Atlantic, Station 332. depth 2200 fathoms.

2. Theopera pyramis, Haeckel.

Shell three-sided pyramidal, with three broad and long hyaline wings, which are prolonged from the collar stricture almost to the basal end, with three diverging edges. Length of the three joints = 3 : 8 : 11, breadth = 4 : 8 : 9. Cephalis subspherical, with a conical horn of the same length. Thorax hemispherical; abdomen inversely campanulate. Pores in the abdomen twice to three times as broad as in the thorax, subregular, circular. Differs from the preceding species mainly in the broader pyramidal form and the divergence of the wings.

Dimensions.—Length of the three joints, a 0.03, b 0.08, c 0.11; breadth, a 0.04, b 0.08, c 0.09.

Habitat.—Fossil in Barbados.

3. Theopera fusiformis (Pl. 67, fig. 5).

Shell nearly spindle-shaped, rough, with two slight strictures. Length of the three joints = 2 : 3 : 6, breadth = 2 : 5 : 4. Cephalis hemispherical, large, with an oblique conical horn of the same length. Thorax hemispherical; abdomen inversely conical. Pores irregular, roundish, of very different sizes. Three wings short, broad, diverging, triangular, and striated, embracing at the base the lumbar stricture, the lower third of the thorax and the upper third of the abdomen.

Dimensions.—Length of the three joints, a 0.04, b 0.06, c 0.12; breadth, a 0.045, b 0.1, c 0.08.

Habitat.—Central Pacific, Stations 263 to 274, depth 2350 to 2925 fathoms.

4. Theopera luscinia, Haeckel.

Shell slenderly ovate, with distinct collar, but indistinct lumbar strictures. Length of the three joints = 1 : 3 : 6, breadth = 1 : 5 : 5. Cephalis hemispherical, with a slender cylindrical horn of three times the length, bearing a spindle-shaped spinulate top (like a fir-cone). Pores subregular, circular, twice as broad in the abdomen as in the thorax. Three wings arising with broad triangular base, embracing the lower third of the thorax and the upper third of the abdomen, prolonged into three slender diverging spines, as long as the shell.

Dimensions.—Length of the three joints a 0.015, b 0.05, c 0.1; breadth, a 0.02, b 0.07, c 0.07.

Habitat.—Fossil in Barbados.

5. Theopera chytropus, n. sp. (Pl. 67, fig. 6).

Shell slender, ovate, with distinct collar, but indistinct lumbar stricture. Length of the three joints = 1 : 2 : 5, breadth = 1 : 3 : 3. Cephalis hemispherical, with a conical horn of the same length. Pores in the thorax and abdomen of similar shape, irregular, roundish. Three wings with broad triangular base attached at the upper half of the shell, in the lower half free, diverging, with a spinulate knob at the distal end.

Dimensions.—Length of the three joints, a 0.025, b 0.06, c 0.15; breadth, a 0.03, b 0.09, c 0.09.

Habitat.—Central Pacific, Stations 265 to 268, depth 2700 to 2900 fathoms.

6. Theopera cortina, n. sp. (Pl. 67, fig. 8).

Shell slender, ovate, with distinct collar, but indistinct lumbar stricture. Length of the three joints = 1 : 2 : 6, breadth 1 : 4 : 4. Cephalis subspherical, with an elegant pyramidal horn of three times the length, bearing an ovate, spinulate knob. Pores subregular, circular, two or three times as large in the abdomen as in the thorax. The three wings are slightly elevated small ribs on the thorax, arise with broad, triangular, striated bases in the upper half of the abdomen, and are prolonged into three slender diverging feet, which are as long as the shell, and bear a spinulate, ovate knob at the distal end.

Dimensions.—Length of the three joints, a 0.02, b 0.05, c 0.13; breadth, a 0.03, b 0.08, c 0.08.

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

Genus 598. Rhopalocanium,^[215] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

Definition.—Theoperida (vel Tricyrtida triradiata clausa) with three lateral wings on the sides of the inversely conical abdomen, which bears no vertical terminal horn on the basal apex.

The genus Rhopalocanium, and the closely allied Rhopalatractus which follows, differ from the preceding Theopera, their probable ancestral form, in the origin of the three lateral wings from the inversely conical abdomen, the thorax bearing no wings. The upper part of the wing bases, which arose formerly from the thorax, is here reduced and lost.

1. Rhopalocanium lasanum, n. sp. (Pl. 67, fig. 10).

Shell subovate, with two distinct strictures. Length of the three joints = 1 : 2 : 3, breadth = 1 : 3 : 4. Cephalis hemispherical, with a conical, dimpled horn of twice the length. Thorax subconical, abdomen campanulate, both with irregular, circular pores. Three wings cylindrical, divergent, about as long as the shell, striated, with an ovate, dimpled knob at the distal end, their broad triangular base being attached to the upper half of the abdomen. (Form rather variable.)

Dimensions.—Length of the three joints, a 0.02, b 0.07, c 0.09; breadth, a 0.03, b 0.09, c 0.12.

Habitat.—Central Pacific, Stations 263 to 268, depth 2650 to 3000 fathoms.

2. Rhopalocanium ornatum, Ehrenberg.

Shell nearly spindle-shaped, with two distinct strictures. Length of the three joints = 1 : 3 : 6, breadth = 1 : 4 : 3. Cephalis hemispherical, with a stout, cylindrical horn of twice the length, bearing a dimpled cone. Thorax ovate, campanulate. Abdomen inversely conical. Pores subregular, circular, twice as large in the thorax as in the abdomen. Three wings compressed, slender, divergent, slightly longer than the abdomen, bearing an ovate, dimpled cone at their distal end, with the broad, triangular, striated base attached to the upper third of the abdomen.

Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.12; breadth, a 0.025, b 0.08, c 0.07.

Habitat.—Fossil in Barbados.

3. Rhopalocanium cortinium, n. sp.

Shell slender, ovate, nearly of the same form as, and with similar fenestration to that of Theopera cortina, (Pl. 67, fig. 8). It differs from the latter mainly by the absence of the three thoracic ribs, characteristic of Theopera. Length of the three joints = 1 : 2 : 6, breadth = 1 : 4 : 4. The three wings arise by a broad, triangular, striated base from the upper half of the abdomen, below the lumbar stricture. The cephalic horn and the three divergent wings are shorter, and at the distal end not so much thickened as in Theopera cortina.

Dimensions.—Length of the three joints, a 0.025, b 0.045, c 0.12; breadth, a 0.03, b 0.07, c 0.08.

Habitat.—Tropical Atlantic, Station 347, depth 2250 fathoms.

4. Rhopalocanium pythia, n. sp.

Shell nearly ovate, with two deep strictures. Length of the three joints = 1 : 4 : 8, breadth = 1 : 4 : 4. Cephalis subspherical, with a conical horn of three times the length. Thorax inflated, abdomen inversely campanulate and prolonged into a short, conical, latticed tube. Pores subregular, circular. Three wings slender, as long as the abdomen, arising by a broader base from the uppermost part of the abdomen, their thickened end being curved inwards.

Dimensions.—Length of the three joints, a 0.02, b 0.07, c 0.15; breadth, a 0.03, b 0.08, c 0.07.

Habitat.—Fossil in Barbados.

5. Rhopalocanium delphicum, n. sp. (Pl. 67, fig. 9).

Shell nearly spindle-shaped, with two deep strictures. Length of the three joints = 1 : 2 : 4, breadth = 1 : 3 : 3. Cephalis hemispherical, with a slender, conical, curved horn, as long as the hemispherical thorax. Abdomen inversely campanulate, prolonged into a short, conical, latticed tube. Pores subregular, circular. Three wings arising by a narrow base from the uppermost part of the abdomen, slender, cylindrical, S-shaped, curved, divergent, with a thick scaly appendix like a fir-cone at the distal end.

Dimensions.—Length of the three joints, a 0.025, b 0.07, c 0.14; breadth, a 0.03, b 0.09, c 0.08.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

Genus 599. Rhopalatractus,^[216] Haeckel, 1881, Prodromus, p. 437.

Definition.—Theoperida (vel Tricyrtida triradiata clausa) with three lateral wings on the sides of the inversely conical abdomen, which bears a vertical terminal horn on the basal apex.

The genus Rhopalatractus, one of the most remarkable forms of Tricyrtida, differs from the preceding Rhopalocanium, its ancestral form, in the production of a vertical, basal horn, which descends from the basal apex of the inversely conical abdomen, and is opposed to the upper apical horn of the cephalis. The shell becomes here, therefore, exquisitely spindle-shaped.

1. Rhopalatractus pentacanthus, n. sp. (Pl. 68, fig. 11).

Shell very thick-walled, with two sharp strictures. Length of the three joints = 1 : 6 : 8, breadth = 1 : 6 : 7. Cephalis hemispherical, with conical cavity, and a conical horn of twice the length. Thorax subspherical, separated from the abdomen by a deep lumbar stricture, with small, regular, circular, hexagonally-framed pores. Abdomen three-sided prismatic, with three strong, prominent, wing-shaped edges, which are slightly divergent towards the base, and prolonged into three pyramidal feet about as long as the thorax. Pores of the abdomen disposed in longitudinal rows, which are separated by denticulate crests. Abdominal base inversely pyramidal, and prolonged into a very large, cylindrical, basal, axial spine, nearly as long as the whole shell.

Dimensions.—Length of the three joints, a 0.02, b 0.12, c 0.15; breadth, a 0.03, b 0.11, c 0.14.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

2. Rhopalatractus foveolatus, n. sp. (Pl. 68, fig. 10).

Shell slenderly ovate, thick-walled, with rough, dimpled surface. Length of the three joints = 1 : 3 : 5, breadth = 2 : 4 : 4. Cephalis hemispherical, with spherical cavity, and a thick, conical spine of the same length. Thorax subspherical, with subregular, circular pores. Abdomen inversely conical, with larger, irregular, roundish pores, prolonged at the distal end into a cylindrical, axial, basal spine of about half its length. From the middle part of the abdomen arise three lateral, conical feet, which are slightly curved and divergent downwardly, scarcely as long as the basal spine. All five spines are dimpled.

Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.1; breadth a 0.04, b 0.08, c 0.08.

Habitat.—Central Pacific, Station 265, depth 2900 fathoms.

3. Rhopalatractus fusiformis, n. sp.

Shell slender, spindle-shaped, nearly of the same form as in the preceding species. Length of the three joints = 1 : 2 : 6, breadth = 1 : 2 : 2. Cephalis subspherical, with a cylindrical horn three times the length. Pores subregular, circular, of equal breadth in the thorax and in the abdomen, which is prolonged into a cylindrical, axial, basal spine of the same length. From the upper third of the abdomen arise, from a broad triangular base, three conical wings of half the length, divergent and slightly curved inwards.

Dimensions.—Length of the three joints, a 0.025, b 0.05, c 0.15; breadth, a 0.03, b 0.06, c 0.06.

Habitat.—Central Pacific, Station 263, depth 2650 fathoms.

4. Rhopalatractus fenestratus, n. sp. (Pl. 68, fig. 12).

Shell nearly spindle-shaped, with two distinct strictures. Length of the three joints = 1 : 3 : 6, breadth = 1 : 4 : 4. Cephalis hemispherical, with a large, club-shaped horn, nearly as long as the abdomen, sulcated by longitudinal ribs, which are elegantly denticulate in the distal half. Thorax subspherical. Abdomen subovate, prolonged into a pyramidal, axial, basal spine, half as long as the thorax. Pores subregular, circular. From the upper half of the abdomen arise three stout, lateral, diverged wings of about the same length, which are curved inwards and fenestrated both at the broad triangular base and at the thickened, three-edged distal end.

Dimensions.—Length of the three joints, a 0.02, b 0.06, c 0.12; breadth, a 0.03, b 0.08, c 0.08.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

Genus 600. Lithochytris,^[217] Ehrenberg, 1847, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 54.

Definition.—Theoperida (vel Tricyrtida triradiata clausa) with a three-sided pyramidal abdomen, the triangular base of which is prolonged at the three corners into three terminal feet.

The genus Lithochytris differs from all the other Theoperida in the absence of lateral wings and the possession of three divergent, terminal feet, which arise from the three corners of the three-sided pyramidal abdomen. It repeats, therefore, among the Tricyrtida, that characteristic formation which Sethochytris and Tetrahedrina represent among the Dicyrtida. Some species may be derived from Pterocanium, other species from Podocyrtis, by the development of a lattice-plate closing the terminal mouth.

Subgenus 1. Lithochytrodes, Haeckel.

Three feet of the abdomen solid, not fenestrated, representing external apophyses of the triangular shell-base.

1. Lithochytris cortina, n. sp. (Pl. 67, fig. 12).

Shell three-sided pyramidal, without external strictures, but with two internal girdles. Length of the three joints = 2 : 2 : 5, breadth = 3 : 4 : 6. Cephalis large, truncate-pyramidal, with nine deep divergent ribs (and nine longitudinal rows of pores between them), and with a stout pyramidal horn of half the length. The three prominent edges of the thorax and abdomen are prolonged over the base of the pyramid into three very stout, solid, subovate feet, about as long as the thorax.

Dimensions.—Length of the three joints, a 0.04, b 0.04, c 0.1; breadth, a 0.06, b 0.08, a 0.12.

Habitat.—Central Pacific, Station 266, depth 2750 fathoms.

2. Lithochytris pyriformis, n. sp. (Pl. 67, fig. 13).

Shell pyramidal, nearly pear-shaped, with two distinct strictures. Length of the three joints = 1 : 1 : 3, breadth = 1 : 2 : 3. Cephalis large, pear-shaped, with a short pyramidal horn of half the length. Thorax and abdomen rounded, without prominent edges. Three basal feet pyramidal, solid, nearly vertical, as long as the thorax. Pores irregular, roundish, in the abdomen large, twice to three times as broad as in the thorax.

Dimensions.—Length of the three joints, a 0.03, b 0.03, c 0.08; breadth, a 0.04, b 0.06, c 0.09.

Habitat.—Tropical Atlantic, Station 348, depth 2450 fathoms.

3. Lithochytris tripodium, Ehrenberg.

Shell pyramidal, nearly pear-shaped, with two distinct strictures. Length of the three joints = 1 : 2 : 4, breadth = 1 : 2 : 5. Cephalis with a horn of the same length, conical. Thorax and abdomen rounded, without prominent edges. Three basal feet conical, solid, strongly divergent, shorter than the thorax. Pores subregular, circular, twice as broad in the abdomen as in the thorax.

Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.08; breadth, a 0.03, b 0.05, c 0.1.

Habitat.—Fossil in Barbados.

Subgenus 2. Lithochytridium, Haeckel.

Definition.—Three feet of the abdomen hollow and fenestrated, representing direct protuberances or corner prolongations of the triangular shell-base.

4. Lithochytris galeata, n. sp. (Pl. 67, fig. 16).

Shell pyramidal, with two sharp strictures. Length of the three joints = 1 : 1 : 4, breadth = 1 : 2 : 4. Cephalis subspherical, with an oblique pyramidal horn of the same length. Thorax somewhat larger. Abdomen inflated, with three sharp, prominent edges, which are prolonged over the shell-base into three pyramidal, hollow, and fenestrated feet, longer than the thorax. Pores subregular, circular, twice as broad in the abdomen as in the thorax.

Dimensions.—Length of the three joints, a 0.025, b 0.025, c 0.1; breadth, a 0.025, b 0.05, c 0.1.

Habitat.—South Atlantic, Station 332, depth 2200 fathoms.

5. Lithochytris pileata, Ehrenberg.

Shell three-sided pyramidal, with deep collar, but indistinct lumbar stricture. Length of the three joints = 2 : 2 : 9, breadth = 3 : 5 : 10. Cephalis with a conical horn of the same length, pear-shaped. Thorax very short and broad. Abdomen inflated, with three sharp, prominent edges, which are prolonged over the shell-base into three pyramidal, hollow and fenestrated feet, the lower edge of which is horizontal. Pores regular, circular, of equal size in the thorax and abdomen.

Dimensions.—Length of the three joints, a 0.02, b 0.02, c 0.09; breadth, a 0.03, b 0.05, c 0.1.

Habitat.—Fossil in Barbados.

6. Lithochytris lucerna, n. sp. (Pl. 67, fig. 14).

Shell three-sided pyramidal, with two indistinct strictures. Length of the three joints = 1 : 2 : 5, breadth = 2 : 3 : 8. Cephalis small, with a horn of the same length, conical. Thorax with subspherical cavity. Abdomen with three rounded, prominent edges, which are prolonged over the shell-base into three conical, hollow, and fenestrated feet, twice as long as the thorax, with a thick, pyramidal, terminal spine. Pores subregular, circular, of equal size in the thorax and abdomen.

Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.1; breadth, a 0.04, b 0.06, c 0.16.

Habitat.—South Pacific, Station 297, depth 1775 fathoms.

7. Lithochytris lanterna, n. sp. (Pl. 67, fig. 11).

Shell three-sided pyramidal, with two distinct strictures. Length of the three joints = 1 : 2 : 4, breadth = 1 : 3 : 6. Cephalis small, with a pyramidal slender horn of twice the length. Thorax with subspherical cavity. Abdomen with three sharp prominent edges, which are prolonged over the convex shell-base into three slender, prismatic, hollow, fenestrated feet, strongly divergent, twice as long as the thorax (seen in fig. 11 from the vaulted base). Pores circular, twice as broad in the abdomen as in the thorax.

Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.08; breadth, a 0.02, b 0.06, c 0.12.

Habitat.—North Pacific, Station 244, depth 2900 fathoms.

8. Lithochytris pyramidalis, Ehrenberg.

Shell three-sided pyramidal, without external strictures, but with two internal transverse girdles. Length of the three joints = 1 : 2 : 6, breadth = 2 : 4 : 8. Cephalis with a short horn of half the length, conical. Cavity of the thorax subspherical. Abdomen with three rounded edges, prolonged over the concave base into three pyramidal, hollow, and fenestrated feet, twice as long as the thorax. Pores large, irregular, roundish.

Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.12; breadth, a 0.04, b 0.08, c 0.16.

Habitat.—Central Pacific, Stations 265 to 268, depth 2700 to 2900 fathoms; fossil in Barbados.

9. Lithochytris pteropus, n. sp. (Pl. 67, fig. 15).

Shell three-sided pyramidal, with two indistinct strictures, but with two broad, internal girdles. Length of the three joints = 2 : 2 : 6, breadth = 2 : 3 : 9. Cephalis with a small horn of half the length, pear-shaped. Thorax broader than long. Abdomen with three prominent, rounded edges, prolonged over the concave base into three conical, hollow, and fenestrated feet, twice as long as the thorax. Pores small, circular, irregular, in longitudinal series along the edges.

Dimensions.—Length of the three joints, a 0.04, b 0.04, c 0.12; breadth, a 0.04, b 0.06, c 0.18.

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.

10. Lithochytris vespertilio, Ehrenberg.

Shell three-sided pyramidal, with two indistinct strictures. Length of the three joints = 1 : 2 : 5, breadth = 2 : 3 : 10. Cephalis with a short horn of half the length, conical. Thorax inflated. Abdomen without prominent edges, divided in the lower half into three large, conical, hollow, and fenestrated feet, twice as long as the thorax. Pores small, irregular, roundish.

Dimensions.—Length of the three joints, a 0.02, b 0.04, c 0.1; breadth, a 0.04, b 0.06, c 0.2.

Habitat.—Fossil in Barbados.

Family LXVI. Phormocyrtida, n. fam.

Definition.—Tricyrtida multiradiata. (Cyrtoidea with a three-jointed shell, divided by two transverse constrictions into cephalis, thorax, and abdomen, with numerous, four to nine or more, radial apophyses.)

The family Phormocyrtida, composed of the Theophormida and Theophænida of my Prodromus, comprises those Cyrtoidea in which the lattice-shell is three-jointed, and bears numerous radial appendages (usually six or nine, sometimes more, rarely less, four or five). The two subfamilies differ in the shape of the terminal mouth, which is in the Theophormida a simple wide opening, in the Theophænida closed by a lattice-plate. The phylogenetic origin of the Phormocyrtida may be found either in the Podocyrtida or in the Anthocyrtida; they may be derived either from the former by interpolation of interradial, secondary apophyses between the three primary perradial apophyses; or from the latter by development of an abdomen.

The radial apophyses are originally radial ribs, which arise from the base of the cephalis on the collar stricture, run along the thorax and abdomen, and are often prolonged into terminal feet. Whilst in some forms the radial ribs are completely preserved in both joints, they are in other forms only partly visible (in the abdomen), and very often only their free terminal prolongations are preserved in the form of a corona of feet around the mouth of the thorax. This corona is either simple or double. Sometimes also a corona is developed on the lumbar stricture, between the thorax and abdomen.

These apophyses exhibit a remarkable variety in the great subfamily Theophormida, with open mouth. In the small subfamily Theophænida, however, the apophyses appear as six or nine simple lateral wings on the abdomen.

The Theophormida are richly represented not only in the present seas, but also as fossils in Barbados, and numerous remarkable forms have been already described by Ehrenberg, in his genera Calocyclas and Cycladophora. Many Phormocyrtida belong to the most elegant and admirable forms of Radiolaria.

Synopsis of the Genera of Phormocyrtida.

Subfamily 1. Theophormida, Haeckel, 1881, Prodromus, p. 436.

Definition.—Phormocyrtida with the basal mouth of the shell open (vel Tricyrtida multiradiata aperta).

Genus 601. Theophormis,^[218] Haeckel, 1881, Prodromus, p. 436.

Definition.—Theophormida (vel Tricyrtida multiradiata aperta) with numerous radial ribs enclosed in the wall of the thorax and of the flat dilated abdomen; mouth of the latter wide open.