Genus 274. Larnacalpis,^[313] n. gen.

Definition.—Larnacida with a simple lentelliptical cortical shell, without radial spines. Medullary shell double, Larnacilla-shaped.

The genus Larnacalpis represents the most simple form of the sub-family Larnacalpida, and is very important as the common original form of all those Larcoidea in which a double Larnacilla-shaped medullary shell is surrounded by a simple, perfectly closed, latticed, lentelliptical cortical shell. Therefore the same typical, trizonal, lentelliptical shell, which in Larnacilla represents the external envelop (or cortical shell) of the central capsule, here in Larnacalpis becomes enclosed as an internal nucleus (or medullary shell) in the interior of the central capsule, and this latter becomes overgrown by a new lentelliptical cortical shell. The connection between the two shells of Larnacalpis is either effected by a number of radial beams (e.g., in Larnacalpis triaxonia by six beams situated in the three dimensive axes), or by two lateral, latticed, tube-like wings, which are repetitions of the smaller lateral wings connecting its external shell with the internal medullary shell (as in Larnacalpis lentellipsis). The latter species may be regarded as a Pylonium with a completely latticed shell.

1. Larnacalpis lentellipsis, n. sp. (Pl. 50, figs. 2, 2a, 2b).

Cortical shell with thorny surface and irregular network; pores roundish, twice to four times as broad as the bars; about sixteen on the half meridian, twelve on the half equator. Proportion of the three dimensive axes = 2 : 3 : 4. Medullary shell one-third as large as the cortical, with four elliptical internal gates, connected with it by two opposite beams in the principal axis and by two latticed wings in the transverse axis; therefore between the two shells are four large kidney-shaped gates, halved by the polar beams (as in Octopyle).

Dimensions.—Length of the cortical shell (or principal axis) 0.14, breadth (or transverse axis) 0.11, height (or sagittal axis) 0.07; pores 0.01 to 0.02, bars 0.005; length of the medullary shell 0.05, breadth 0.04, height 0.03.

Habitat.—Pacific, central area, Station 273, depth 2600 fathoms.

2. Larnacalpis phacodiscus, n. sp.

Cortical shell with thorny surface and regular network; pores circular, twice as broad as the bars; about ten on the half meridian, eight on the half equator. Proportion of the three axes = 2 : 2.5 : 3. Medullary shell half as large as the cortical, with four kidney-shaped internal gates.

Dimensions.—Length of the cortical shell 0.13, breadth 0.11, height 0.09; pores 0.012, bars 0.006; length of the medullary shell 0.06, breadth 0.045, height 0.03.

Habitat.—North Pacific, Station 253, depth 3125 fathoms.

3. Larnacalpis macrococcus, n. sp.

Cortical shell with spiny surface and regular network; pores circular, small, of the same breadth as the bars; about twenty-two on the half meridian, nineteen on the half equator. Proportion of the three axes = 2 : 3 : 4. Medullary shell two-thirds as large as the cortical, with four wide internal semicircular gates.

Dimensions.—Length of the cortical shell 0.1, breadth 0.075, height 0.05; pores and bars 0.003; length of the medullary shell 0.066, breadth 0.05, height 0.032.

Habitat.—Western Tropical Pacific, Station 224, depth 1850 fathoms.

4. Larnacalpis subsphærica, n. sp.

Cortical shell with rough surface and irregular network; roundish pores twice to five times as broad as the bars; about twenty-four on the half meridian, twenty-one on the half equator. Proportion of the three axes = 1.2 : 1.3 : 1.4. Medullary shell half as large as the cortical, with four elliptical internal gates.

Dimensions.—Length of the cortical shell 0.14, breadth 0.13, height 0.12; pores 0.004 to 0.01, bars 0.002; length of the medullary shell 0.08, breadth 0.07, height 0.06.

Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.

5. Larnacalpis triaxonia, n. sp. (Pl. 50, fig. 3).

Cortical shell with smooth surface and peculiar network, composed of four meridian rows of larger pores (five large elliptical pores on each half meridian, the largest in the equator) and numerous small irregular pores between them. Proportion of the three axes = 2 : 3 : 4. Medullary shell with four semicircular internal gates, about one-fourth as large as the cortical, connected with it by six thin radial beams, opposite in pairs in the three dimensive axes.

Dimensions.—Length of the cortical shell 0.14, breadth 0.1, height 0.07; large pores 0.03, small pores 0.002 to 0.01, bars 0.002 to 0.01; length of the medullary shell 0.04, breadth 0.03, height 0.02.

Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.

Genus 275. Larnacantha,^[314] n. gen.

Definition.—Larnacida with a simple lentelliptical cortical shell, armed with symmetrically disposed radial spines. Medullary shell double, Larnacilla-shaped.

The genus Larnacantha has the same characteristic shell-formation as the foregoing Larnacalpis, and differs from it only in the possession of radial spines, which are symmetrically distributed on the surface in a definite order. Commonly these spines are external prolongations of the internal radial beams, which connect the double Larnacilla-shaped medullary shell with the simple lentelliptical cortical shell.

1. Larnacantha dissacantha, n. sp.

Cortical shell smooth, with two large cylindrical spines, opposite on the poles of the principal axis, and somewhat longer than it. Pores regular, circular, three times as broad as the bars; about eleven on the half meridian. Proportion of the three axes = 2 : 3 : 4. Medullary shell one-third as large as the cortical shell.

Dimensions.—Length of the cortical shell (principal axis) 0.13, breadth (transverse axis) 0.1, height (sagittal axis) 0.07; pores 0.009, bars 0.003; length of the Larnacilla-shaped medullary shell 0.045.

Habitat.—North Pacific, Station 256, depth 2950 fathoms.

2. Larnacantha stauracantha, n. sp.

Cortical shell smooth, with four large conical spines in the lateral plane, two larger opposite on the poles of the principal, two smaller on those of the transverse axis. Pores regular, circular, twice as broad as the bars; about seventeen on the half meridian. Proportion of the three axes = 3 : 3.5 : 4. Medullary shell one-third as large as the cortical shell.

Dimensions.—Length of the cortical shell 0.15, breadth 0.13, height 0.11; pores 0.008, bars 0.004; length of the medullary shell 0.05.

Habitat.—Pacific, central area, Station 274, surface.

3. Larnacantha quadricornis, n. sp.

Cortical shell spiny, with four strong, horn-like curved spines in the lateral plane, opposite in pairs on the poles of the crossed diagonal axes. Pores irregular, roundish, twice to four times as broad as the bars; about fourteen on the half meridian. Proportion of the three axes = 2 : 3 : 4. Medullary shell one-third as large as the cortical shell.

Dimensions.—Length of the cortical shell 0.14, breadth 0.11, height 0.07; pores 0.006 to 0.012, bars 0.003; length of the medullary shell 0.05.

Habitat.—Pacific, central area, Station 270, surface.

4. Larnacantha hexacantha, n. sp. (Pl. 50, fig. 4).

Cortical shell thorny, with six strong conical radial spines in the lateral plane, two opposite on the poles of the principal axis, four others opposite in pairs on the poles of the two crossed diagonal axes. Pores with peculiar distribution; twelve large elliptical pores (nearly of the size of the medullary shell) symmetrically disposed in four crossed meridians (between the sagittal and the lateral meridians), separated by bands of smaller irregular pores. Proportion of the three axes = 3 : 4 : 5. Medullary shell hexagonal, one-third as large as the cortical shell.

Dimensions.—Length of the cortical shell 0.14, breadth 0.11, height 0.08; large pores 0.04, small pores 0.003 to 0.01, bars 0.004; length of the medullary shell 0.05.

Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.

5. Larnacantha bicruciata, n. sp. (Pl. 50, fig. 5).

Cortical shell thorny, in the lateral plane with eight strong conical radial spines, alternating with eight smaller spines; four of the eight stronger spines opposite on the poles of the principal and transverse axes (in the figure 5, by mistake, not represented large enough), four others between those, opposite on the poles of the two crossed diagonal axes. Pores with a peculiar disposition; on both flat sides of the lentellipsis an elliptical ring of eight large elliptical pores (alternating with the eight stronger radial spines), separated by bands of smaller irregular pores. Proportion of the three axes = 2 : 3 : 4. Medullary shell nearly half as long as the cortical shell.

Dimensions.—Length of the cortical shell 0.16, breadth 0.13, height 0.08; large pores 0.03, small pores 0.003 to 0.01, bars 0.006; length of the medullary shell 0.07.

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

6. Larnacantha octacantha, n. sp.

Cortical shell thorny, with eight long and thin, cylindrical, radial spines, lying opposite in pairs in two crossed diagonal planes. Pores irregular, roundish, twice to five times as broad as the bars; about sixteen on the half meridian. Proportion of the three axes = 1 : 1⅓ : 2. Medullary shell scarcely one-fourth as long as the cortical shell. (This species resembles closely the common Tetrapyle octacantha, from which it seems to be developed by a complete over-growing of the four gates, which become closed by a network connecting the free edges of the transverse and lateral girdles.)

Dimensions.—Length of the cortical shell 0.18, breadth 0.13, height 0.1; pores 0.005 to 0.015, bars 0.003; length of the medullary shell 0.04.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

7. Larnacantha cladacantha, n. sp.

Cortical shell very spiny, with eight longer ramified spines, lying opposite in pairs in two crossed diagonal planes; each spine with two to six irregular, lateral branches. Pores irregular, roundish, twice to three times as broad as the bars; about twelve on the half meridian. Proportion of the three axes = 5 : 6 : 7. Medullary shell nearly half as long as the cortical shell. (Differs from the foregoing by the branching spines end the larger medullary shell.)

Dimensions.—Length of the cortical shell 0.15, breadth 0.13, height 0.11; pores 0.01 to 0.015, bars 0.005; length of the medullary shell 0.07.

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

8. Larnacantha prismatica, n. sp. (Pl. 50, fig. 6).

Cortical shell smooth, four-sided prismatic, with eight short, parallel, three-sided pyramidal spines; these lie opposite in pairs in four parallel longitudinal lines, as prolongations of the four lateral edges of the prism, and arise from its eight corners. Pores regular, circular, three times as broad as the bars; about fourteen on the half meridian. Proportion of the three axes = 2 : 3 : 4. Medullary shell half as long as the cortical shell.

Dimensions.—Length of the cortical shell 0.11, breadth 0.07, height 0.05; pores 0.006, bars 0.002; length of the medullary shell 0.06.

Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.

9. Larnacantha decacantha, n. sp.

Cortical shell thorny, with ten short and stout, conical, radial spines, two opposite on the poles of the principal axis (as prolongations of inner axial beams), eight others opposite in pairs in two crossed diagonal planes. Pores irregular, roundish, twice to four times as broad as the bars; about sixteen on the half meridian. Proportion of the three axes = 3 : 3.75 : 4. Medullary shell about one-third as long as the cortical shell.

Dimensions.—Length of the cortical shell 0.16, breadth 0.15, height 0.12; pores 0.008 to 0.016, bars 0.004; length of the medullary shell 0.06.

Habitat.—North Atlantic, Station 354, surface.

10. Larnacantha dodecantha, n. sp.

Cortical shell nearly smooth, but with twelve strong conical radial spines; four in the lateral plane opposite in pairs (two on the poles of the principal, and two on the poles of the transverse axis); eight others opposite in pairs in two crossed diagonal planes. Pores with a peculiar disposition: twelve large elliptical pores in two crossed meridian planes (alternating with the twelve spines), separated by bands of irregular small pores. Proportion of the three axes = 1 : 1.5 : 2. Medullary shell hexagonal, one-third as long as the cortical shell.

Dimensions.—Length of the cortical shell 0.15, breadth 0.11, height 0.08; large pores 0.03, small pores 0.005 to 0.01, bars 0.003; length of the medullary shell 0.05.

Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.

11. Larnacantha drymacantha, n. sp.

Cortical shell very spiny, on the whole surface covered with a forest of numerous (thirty to fifty or more) large branched spines, about the length of the shell; each spine with three to nine lateral branches, simple or forked (very similar to Cromyodrymus abietinus, Pl. 30, fig. 6). Pores very irregular, roundish. Proportion of the three axes = 2 : 2.5 : 3. Medullary shell half as long as the cortical shell.

Dimensions.—Length of the cortical shell 0.16, breadth 0.13, height 0.1; pores 0.005 to 0.015, bars 0.03; length of the medullary shell 0.08.

Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.

Genus 276. Larnacoma,^[315] n. gen.

Definition.—Larnacida with double lentelliptical cortical shell, without radial spines. Medullary shell double, Larnacilla-shaped.

The genus Larnacoma has originated from the nearly allied Larnacalpis by duplication of the cortical shell. Whilst in both genera the connection between the Larnacilla-shaped medullary shell and the primary cortical shell is the same, many short radial beams arise from the surface of the latter in Larnacoma, which at constant equal distances from it unite by a network forming the secondary or outer cortical shell. It differs from the similar Druppulida (Cromyodruppa) by the sagittal flattening of the lentelliptical shell and the Larnacilla-form of the double medullary shell.

1. Larnacoma lentellipticum, n. sp.

Shell with smooth surface and elliptical perimeter, one and a third times as long as broad. All four shells lentelliptical. Distance between the two cortical shells about twice as great as the distance of the inner cortical and outer medullary shell. Network of both outer shells irregular, with large roundish pores, twice to six times as broad as the bars.

Dimensions.—Length (or principal axis) of the first (innermost) shell 0.03, second 0.08, third 0.16, fourth (outermost) 0.27, breadth (or transverse axis) corresponding—(A) 0.02, (B) 0.05, (C) 0.11, (D) 0.2.

Habitat.—South Atlantic, Station 323, depth 1900 fathoms.

2. Larnacoma quadruplex, n. sp.

Shell with thorny surface and elliptical perimeter, one and a fifth times as long as broad. All four shells lentelliptical. Distance between the two cortical shells somewhat smaller than the distance between the inner cortical and outer medullary shell. Network of both outer shells irregular, with large roundish pores, twice to ten times as broad as the bars.

Dimensions.—Length of the first shell 0.02, second 0.06, third 0.15, fourth 0.24; breadth corresponding—(A) 0.016, (B) 0.04, (C) 0.11, (D) 0.2.

Habitat.—South Atlantic, Station 335, depth 1425 fathoms.

3. Larnacoma hexagonium, n. sp.

Shell with thorny surface and hexagonal perimeter, as long as broad. All four shells hexagonal, connected by six piercing radial beams (two in the principal axis, four others in two crossed diagonals). Distance between the two cortical shells somewhat greater than the distance between the inner cortical and outer medullary shell. Network of both outer shells subregular, with small circular pores, twice as broad as the bars.

Dimensions.—Length of the first shell 0.02, second 0.05, third 0.09, fourth 0.16; breadth corresponding—(A) 0.015, (B) 0.035, (C) 0.065, (D) 0.12.

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

Genus 277. Larnacospongus,^[316] n. gen.

Definition.—Larnacida with spongy lentelliptical cortical shell, without radial spines. Medullary shell double, Larnacilla-shaped.

The genus Larnacospongus differs from the nearly allied genera Larnacalpis and Larnacoma by the spongy texture of the lentelliptical cortical shell whilst the enclosed medullary shell in both genera is the same trizonal Larnacilla-shell. Therefore Larnacospongus (and the following nearly related Larnacostupa) can be derived directly by development of a spongy envelop either from Larnacilla and Larnacalpis, or from Trizonium and Amphipyle. But some species of these spongy genera appear to be derived rather from Tetrapyle or Pylonium, perhaps also from Cubotholus. Their phylogenetic origin may be explained in different ways.

1. Larnacospongus larnacillifer, n. sp.

Cortical shell lentelliptical, one and a half times as long as broad, with rough surface and rather loose spongy framework, directly enclosing a trizonal Larnacilla-shell of the same form, but of only one-third its size.

Dimensions.—Length of the whole shell 0.17, breadth 0.12; length of the medullary shell 0.06, breadth 0.04.

Habitat.—South Atlantic, east coast of Patagonia, Station 319, surface.

2. Larnacospongus tetrapylifer, n. sp.

Cortical shell lentelliptical, one and a third times as long as broad, with thorny surface; composed of an outer envelop of loose spongy framework and an inner lattice-shell with four kidney-shaped gates, like Tetrapyle; the latter encloses a trizonal medullary shell of one-fourth its size.

Dimensions.—Length of the whole shell 0.22, breadth 0.16; length of the medullary shell 0.045, breadth 0.035.

Habitat.—South Atlantic, west of Tristan da Cunha, Station 332, surface.

Genus 278. Larnacostupa,^[317] n. gen.

Definition.—Larnacida with spongy lentelliptical cortical shell, with radial spines on the surface. Medullary shell double, Larnacilla-shaped.

The genus Larnacostupa differs from the preceding Larnacospongus only in the possession of radial spines, covering either the whole surface irregularly or disposed in a certain symmetrical order.

1. Larnacostupa octacantha, n. sp.

Cortical shell lentelliptical, with thorny surface, and rather dense, irregular, spongy framework, which arises from an inner latticed cortical shell, like that of Tetrapyle or Pylonium. This latter is twice as large as the enclosed Larnacilla-shell. Eight long and thin, cylindrical, radial spines, opposite in pairs in two diagonal planes. (Seems to be the common Tetrapyle octacantha, enveloped by an outer spongy framework mantle.)

Dimensions.—Length of the whole shell (without spines) 0.22, breadth 0.16; length of the medullary shell 0.06, breadth 0.04.

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

2. Larnacostupa spinosa, n. sp.

Cortical shell lentelliptical, about one and a half times as long as broad, with very lax and irregular spongy framework, arising from a nearly quadrangular lattice-shell like that of Tetrapyle; this latter encloses a Larnacilla-shell of half its size. Whole surface covered with thin bristle-like radial spines, of about the length of the shell.

Dimensions.—Length of the whole shell (without spines) 0.18, breadth 0.12; length of the medullary shell 0.05, breadth 0.035.

Habitat.—Antarctic Ocean, off Kerguelen, Station 150, surface.

3. Larnacostupa dendrophora, n. sp.

Cortical shell nearly spherical, scarcely longer than broad, with lax, irregular spongy framework, arising from a lentelliptical trizonal medullary shell (like Larnacilla). Whole surface covered with thin arborescent radial spines, about half as long as the shell, each spine with three to six irregular branches.

Dimensions.—Length of the whole shell (without spines) 0.15, breadth 0.13; length of the medullary shell 0.07, breadth 0.04.

Habitat.—Antarctic Ocean, Station 154, surface.

Family XXVI. Pylonida, Haeckel, 1881 (Pl. 9).

Pylonida, Haeckel, 1881, Prodromus, p. 463.

Definition.—Larcoidea with regular, incompletely latticed cortical shell, distinguished by two to four or more symmetrically disposed gates or large fissures remaining between one to three latticed dimensive girdles (perpendicular one to another). One, two, or three concentric systems of such girdles (each system with three girdles) may be developed.

The family Pylonida is the most important and interesting among all the Larcoidea, not only because it is much richer in different and peculiar forms than the other families of this section, but also because it has direct and very complex relations to all the other families of Larcoidea. It is even possible that the Pylonida represent the original ancestral group of the whole section, and that the apparently simpler group of the Larcarida must be derived from the former by retrogressive metamorphosis.

Till the year 1881 the family Pylonida, which here now exhibits ten genera with eighty-six species, was only represented by one single species, accurately described and extensively illustrated by Johannes Müller in 1858, the well known and widely distributed cosmopolitan Tetrapyle octacantha (Abhandl. d. k. Akad. d. Wiss. Berlin, p. 33, Taf. iii.). A slight modification of it was afterwards described by Ehrenberg as Schizomma quadrilobum (Abhandl. d. k. Akad. d. Wiss. Berlin, 1872, Taf. ii. fig. 12). A more accurate description of it, with a good explanation of its characteristic growth, was given in 1879 by Richard Hertwig in his Organismus der Radiolarien (pp. 52-54, Taf. iv. figs. 7, 8; Taf. vi. figs. 2, 5). In my Prodromus (1881, p. 463) I constituted for a large number of allied species, detected in the Challenger collection, the special family Pylonida, and distinguished among it twelve different genera. However, I think it now better to restrict the definition of the family as given in the above definition, and to remove from it a number of genera formerly with it united, as the genera Triopyle and Hexapyle, appertaining to the Discoidea.

The characteristic type of all true Pylonida is clearly demonstrated by their peculiar mode of growth, the consequence of which is the imperfect lattice-work of the fenestrated larcoid shell. This remarkable growth is effected by the development of elliptical latticed girdles (or rings), which enclose a quite simple, spherical, subspherical, or lentelliptical primordial shell. The girdles lie in three different planes, perpendicular to one another, and are of different sizes; each girdle being somewhat larger than the foregoing and somewhat smaller than the following girdle. Between these latticed girdles remain on the surface of the shell large openings or "gates," which are not closed by network, and it is just the symmetrical disposition and form of these open "gates," separated and enclosed by the fenestrated girdles, which give to the Pylonida their characteristic appearance.

To understand clearly this peculiar constitution of the Pylonida-shell by a system of alternating girdles, developing one after the other it is indispensable to pay careful attention to the three different elliptical dimensive planes, which characterise all Larcoidea, and to the three different dimensive axes, which bisect those planes. The girdle which first develops around simple primordial shell or central chamber is the transverse girdle, lying in the equatorial plane; then comes, secondly, the lateral girdle, lying in the lateral plane; and thirdly follows the sagittal girdle, lying in the sagittal or median plane. The three simplest genera of the Pylonida—Monozonium, Dizonium, Trizonium—represent these three different stages, with one, two, or three girdles. These three genera constitute the first subfamily, Haplozonaria (with one single system of girdles); all three girdles lie in the surface of a simple lentelliptical cortical shell.

From this first subfamily the other two subfamilies of Pylonida must be derived, by repetition of the same characteristic process of growth. In the Diplozonaria a second system of girdles has been developed, constituting a second (outer) cortical shell of lentelliptical form, concentric with the first. Also in this second system the transverse girdle is first developed, secondly the lateral girdle, thirdly the sagittal girdle. The three genera Amphipyle, Tetrapyle (with Octopyle), and Pylonium represent these three different stages of growth.

Commonly the growth of the Pylonida stops with the completion of the second system but sometimes the same process is once repeated and a third system of girdles is formed, constituting a third lentelliptical shell; in this case also the succession of the three latticed girdles is the same; firstly the (third) transverse girdle is formed, secondly the (third) lateral girdle, and thirdly the (third) sagittal girdle. Each of these three girdles of the third system encloses concentrically the corresponding girdles of the second and first system. The three corresponding genera of this third subfamily (Triplozonaria) are Amphipylonium, Tetrapylonium, and Pylozonium. But in general this highest number of girdles (nine) is very seldom reached; commonly the growth of the Pylonida stops with five girdles (Tetrapyle and Octopyle). More than nine girdles I have never observed, though there remains the possibility of the apposition of a fourth system owing to the peculiar imperfect character of the growth itself.

The central or primordial chamber of the shell, with which in all Pylonida the shell-building commences, is a quite simple, very small fenestrated shell. Commonly one sees on the surface only five to ten small pores (three to four on the diameter). Its form seems to be sometimes spherical, sometimes elongated, ellipsoidal or probably lentelliptical. It may be originally a small Cenolarcus. This simple central chamber, the true "medullary shell" of the small Haplozonaria, is quite different from the medullary shell of the larger Diplozonaria, and particularly of the well-known Tetrapyle. The former observers, J. Müller as well as R. Hertwig, have described in these forms also the medullary shell as a simple spherical or oblong body. But a careful comparison of many hundred specimens of them and of their dimensions, has convinced me that this was an error, and that the small spherical or elliptical medullary shell of Tetrapyle and the other Diplozonaria possesses already the same complex structure, composed of a system of three girdles, as Trizonium and Larnacilla. Whilst in the Haplozonaria probably the simple central chamber only represents the medullary shell (enclosed in the central capsule), and the first system of girdles (complete in Trizonium) the external cortical shell, with the progressive growth this latter becomes enclosed in the central capsule and so constitutes the "trizonal medullary shell" of the Diplozonaria and Triplozonaria.

A very difficult matter is the mode of connection between the cortical and medullary shell. In most of the Pylonida it seems that the first or transverse girdle (in each system) is produced by the formation of two lateral wings or chambers (one on each side of the medullary shell), so that each wing (or half girdle) represents a short and wide, nearly cylindrical tube, the axis of which (with free openings on both poles) is parallel to the principal axis of the medullary shell. In this case (probably the ordinary one) both principal faces of the medullary shell itself (dorsal and ventral face) constitute the middle part of the first girdle whilst its lateral parts are formed by the wings (comparable to the lateral chambers of Amphitholus).

In the second case (probably a much rarer one) there is a free ring-shaped space between the medullary shell and the first (transverse) girdle, and both are connected by a small number of very short and small radial beams (R. Hertwig, loc. cit., p. 52, line 19 to 21 from above). This mode of connection would be the same as is common between the concentric shells of the Sphæroidea and Prunoidea. The distinction between these two different modes of connection is often very difficult.

The second or lateral girdle is commonly not in direct connection with the medullary shell, or only by some scattered radial beams (mainly in the principal axis). This lateral guide arises by prolongation of both wings of the transverse girdle in the lateral plane, so that from both sides (right and left) they become united on the poles of the principal axis. The minor axis of the elliptical lateral ring (thus formed) is therefore the major axis of the foregoing (transverse) elliptical ring; the major axes of both are perpendicular one to another. The major axis of the lateral ring is the principal (or longitudinal) axis of the whole body.

The third or sagittal girdle becomes developed from the second almost in the same manner as the second from the first. On both poles of the principal axis two latticed wings arise from the lateral girdle, growing further in the direction of an elliptical ring, which represents the perimeter of the sagittal plane or median plane. These wings are already mentioned by J. Müller as "prominent roofs, protecting the gates of the Tetrapyle-shell." If these roofs grow towards the equatorial plane of the shell and become united in pairs on the poles of the sagittal axis, the third girdle becomes complete. R. Hertwig supposes that the minor axis of this sagittal girdle is constantly at the same time the major axis of the lateral girdle, but this is not always the case. Very often the size of both these girdles is nearly the same, or one is not much larger than the other. In this case the principal axis of the body is the major axis of the second as well as of the third girdle.

The characteristic "gates" of the Pylonida, or the large wide openings in their cortical shell, remaining between the crossed latticed girdles, are in general roundish, sometimes nearly circular, commonly more elliptical, kidney-shaped or semilunar, their special form varying much according to the different form of the girdles. The narrowest part of each girdle, or its "isthmus," in the case of the halves of the transverse girdle is commonly at their origin from the medullary shell, in the case of the halves of the lateral girdle at the poles of the principal axis, and in the case of the halves of the sagittal girdle at the poles of the sagittal axis. The number of the gates is quite constant in the different genera. If only one girdle (the transverse) be developed, we find only two large gates, between the two wings on the poles of the principal axis (in Monozonium, Amphipyle, Amphipylonium). In all other cases there are four gates (determining the original name "Tetrapyle"), as well if only two or if all three girdles be completed. If two girdles be complete (in Dizonium, Tetrapyle, Tetrapylonium) the four gates lie opposite in pairs on the sagittal faces (two anterior and two posterior gates), and are limited by the transverse and lateral girdles. If all three girdles be complete (in Trizonium, Pylonium, Pylozonium) the four gates lie opposite in pairs on the lateral faces (two dorsal and two ventral gates), and are limited by the sagittal and the transverse girdles. If we turn the shell through an angle of 90°, we have the same aspect as in the former group. A sagittal septum sometimes becomes developed, beginning with two polar beams, rising from the poles of the principal axis. If these polar beams become branched and connected with the middle part of the lateral girdle, we get a latticed vertical septum, which divides the four gates of Tetrapyle into eight gates, Octopyle.

The lattice-work of the Pylonida is commonly very variable and irregular, with roundish meshes of very unequal size, therefore without value in the determination of the species. Commonly the outside of the shell is thorny, and often distinguished by larger radial spines, symmetrically disposed. We can separate these into two groups; "dimensive" spines, lying in one of the three dimensive axes (principal, transverse, or sagittal), and "diagonal" spines, lying crossed in pairs in diagonal axes. Among these latter eight diagonal wing-spines, which arise from the lateral edges of the four gates, are particularly remarkable; they are not only characteristic of Tetrapyle octacantha, but also of a large number of other Pylonida, and form the starting-point for many specific forms.

The shell of the Pylonida is characterised by extraordinary variability and great inclination to individual abnormalities, formation of varieties and transitions into other families, hence derived, as Larnacida, Tholonida, Lithelida, &c.

The central capsule in all Pylonida, in which I could observe it, was a true lentellipsis (or a "triaxial ellipsoid" in the geometrical sense, with the three unequal isopolar axes of the "rhombic octahedron"). In the living Pylonida it is commonly coloured pink or scarlet. During growth its dimensions are probably more or less changed, and perhaps the axes alternate. Regarding the relation of the central capsule to the skeleton, we can distinguish two different groups, quite as in the nearly allied Larnacida. In the Haplozonaria (as also in the Larnacillida) the central capsule encloses only the central chamber and is enveloped by the first system of girdles, whereas in the Diplozonaria and Triplozonaria (as in the Larnacalpida) that "trizonal shell" becomes enclosed (as the "medullary shell") in the central capsule, which is now enveloped by the second system of girdles as the "cortical shell."

Synopsis of the Genera of Pylonida.

Subfamily 1. Haplozonaria, Haeckel.

Definition.—Pylonida with one single system of fenestrated girdles (with one, two, or three girdles, lying in one lentelliptical face).