2. Acantholonche peripolaris, n. sp. (Pl. 132, fig. 8).

Two principal spines quadrangular prismatic, with four broad prominent lamellar wings, of increasing breadth towards the pyramidal distal apex. Both ends of each spine four-sided pyramidal, base without leaf-cross. Two transverse and eight tropical spines about two-thirds as long as the former, four-sided pyramidal in the basal half, conical in the distal half, often curved. Eight polar spines very small, about one-fourth as long as the latter, short conical or pyramidal. Central capsule four-sided prismatic, enveloping both principal spines.

Dimensions.—Length of the two principal spines 0.2, of the ten smaller spines 0.12, of the eight rudimentary polar spines 0.04.

Habitat.—Central Pacific, Station 274, surface.

Order IV. ACANTHOPHRACTA, Richard Hertwig, 1879.

Definition.—Acantharia with complete latticed shell.

The order Acanthophracta, the fourth order of Radiolaria, comprises all those Acantharia in which the acanthinic skeleton is a complete latticed or fenestrated shell, supported by radial spines arising from one common central point. By the possession of such a complete shell the Acanthophracta differ from their ancestral group, the nearly allied Acanthometra, which represent the older and simpler, first order of Acantharia. All Acanthophracta are Icosacantha (like the Acanthonida, their ancestral group), and possess twenty radial spines disposed according to the Müllerian law (compare above, p. 717).

Johannes Müller, who first observed five representatives of this order, called a part of them "Acanthometræ cataphractæ," and united these with the true Acanthometra (Acanthometra costata and Acanthometra cataphracta; Abhandl. d. k. Akad. d. Wiss. Berlin, 1858, pp. 12, 49). Another part was united by him with the true Haliomma (Haliomma echinoides, Haliomma hystrix, Haliomma tabulatum; Abhandl. d. k. Akad. d. Wiss. Berlin, 1858, pp. 36, 37). He supposed that these latter formed the immediate transition from the true Acanthometra to the true Haliomma, and that their skeleton was siliceous.

In my Monograph (1862, p. 412) I founded a separate subfamily, Dorataspida, for the "Acanthometræ cataphractæ," which I considered as the first subfamily of the "Ommatida." That subfamily contained at that time only two genera, Dorataspis (with seven species) and Haliommatidium (with five species). A third genus, Aspidomma (with two species), was united by me with the Haliommatida (because of its double shell). For a fourth genus (Diploconus) with a single species I founded the peculiar family of Diploconida. Therefore the whole number of Acanthophracta described in my Monograph amounted only to four genera and fifteen species. Now the rich collections of the Challenger have added such a great number of new forms, that we may distinguish here thirty-eight genera and two hundred and twelve species.

Richard Hertwig in his excellent work (Der Organismus der Radiolarien, 1879, p. 25) separated his "Acanthophractida" perfectly from the "Ommatida" (or the siliceous Sphæroidea), and united them with the "Acanthometrida" in his order "Acanthometrea." But he separated them also from the nearly allied Diploconida, following my former arrangement. He distinctly noted that the skeleton in all these Acanthophractida (as well as in the Acanthometrida) consists not of silex but of the organic substance "acanthin."

The astonishing number of new and interesting forms of Acanthophracta which I have found in the rich collection of the Challenger enables me to distinguish now in this suborder six different families, two of which are perfectly new (the Sphærocapsida and the Hexalaspida). But the four other families also are so much enlarged that their interesting morphology appears in quite a new and clear light. Far the largest and most important of these six families is that of the true Dorataspida, which embraces seventeen genera and one hundred and eight species (more than the other five families together). From this largest and oldest ancestral family four other families have afterwards arisen, whilst a single family, the Sphærocapsida, seems to possess no direct phylogenetic connection with the five other families.

The peculiar and quite new family of Sphærocapsida (Pl. 133, figs. 7-11; Pl. 135, figs. 6-10) differs from all other Acanthophracta in the singular structure of the spherical acanthinic shell, composed of innumerable small plates or aglets, each of which is pierced by a very small porule. This peculiar pavemented shell (enclosing the central capsule and separated from it by the jelly-like calymma) seems to be produced on the surface of the spherical calymma, immediately by secretion of the pseudopodia, and independently from the twenty radial spines, united in the centre of the sphere. On the twenty points, where the spines perforate the shell, there are originally eighty larger pores (four around each piercing spine); but there is no certain indication that the shell is produced by the meeting apophyses of the twenty spines, as is the case in the five other families of Acanthophracta. Therefore perhaps it is more natural to unite these latter into another suborder as Cladophracta, and to separate them from the Sphærocapsida, which may be called Capsophractæ.

The Dorataspida (Pls. 134-138), the common ancestral stock of the Cladophracta, in the definition here restricted embraces all those Acanthophracta in which the spherical lattice-shell is simple and composed of the meeting branches of twenty radial spines united in its centre. As already pointed out above, this family is probably diphyletic, and embraces two subfamilies which have been derived originally from two different forms of Acanthonida—the Diporaspida (with two opposite apophyses on each spine) derived from the Phractacanthida, and the Tessaraspida (with four crossed apophyses on each spine) derived from the Stauracanthida; in the former we find originally forty apophyses, in the latter eighty apophyses, by the meeting branches of which the spherical lattice-shell originates. The four following families of Acanthophracta have probably been derived from the Diporaspida.

The Phractopeltida (Pl. 133, figs. 1-6) differ from all other Acanthophracta in the possession of a double lattice-shell, composed of two concentric spheres which are united by the twenty radial spines meeting in the centre. As all Phractopeltida possess originally only two apophyses on each radial spine, they must be derived from the Diporaspida (Orophaspis), and bear to them the same relation as the Dyosphærida do to the Monosphærida. As the spherical central capsule of the Phractopeltida is enclosed between both shells, smaller than the outer, larger than the inner shell, the latter may be called "medullary shell," the former "cortical shell." This family represents among the Acanthophracta only the "Diplophracta," whilst all others are "Haplophracta."

The three families here characterised may be called together "Sphærophracta," as their central capsule and the enveloping shell are constantly spherical (or the shell sometimes an "endospherical polyhedron"). On the contrary the following three families of Acanthophracta may be united as "Prunophracta," as their central capsule and shell are never spherical, but either ellipsoidal or lenticular or of another form. The common ancestral stock of this suborder are the Belonaspida, in which the form of the central capsule and the enclosing lattice-shell is ellipsoidal; they are derived from the Dorataspida (and probably all from the subfamily Diporaspida) by the prolongation of two opposite radial spines which are larger than the eighteen others; they are the two equatorial spines of the "hydrotomical axis" (compare above, p. 719, and Pl. 136, figs. 6-9).

The Hexalaspida (Pl. 139) represent a new and very remarkable family, distinguished from all other Acanthophracta by the preponderating development of six stout radial spines, which are much larger than the fourteen others. These six principal spines lie in one meridian plane of the shell (in the "hydrotomical plane," p. 720), and are the two opposite equatorial spines and the four appertaining polar spines of the same plane. As the fourteen smaller spines develop their apophyses at smaller distances from the centre, the shell assumes a peculiar lenticular or discoidal form, and the margin of this disk bears the six larger spines. Moreover the enclosed small central capsule is lenticular. The Hexalaspida may be derived immediately from the Belonaspida.

The Diploconida (Pl. 140) form the last and the most modified family of all Acanthophracta. The remarkable shell exhibits the strange form of a double cone, bearing in its axis two very large opposite spines; these are the two equatorial spines of the "hydrotomical axis" (p. 719). The double-conical or nearly cylindrical shell is composed of three different parts or segments; the small middle part is the true lattice-shell of the Hexalaspida and Belonaspida, and bears the eighteen smaller (often quite rudimentary) radial spines. The two other parts (opposite on both poles of its hydrotomical axis) are the conical or cylindrical, solid, basal sheaths of the two large equatorial spines, enveloping their major part. In consequence of this peculiar metamorphosis of the shell the Diploconida represent the last and the most aberrant group of all Acantharia.

Synopsis of the Suborders and Families of Acanthophracta.

Suborder I. SPHÆROPHRACTA, Haeckel.

Definition.—Shell spherical, with twenty radial beams of equal size.

Family XXXIX. Sphærocapsida, Haeckel (Pl. 133, figs. 7-11; Pl. 135, figs. 6-10).

Sphærocapsida, Haeckel, 1881, Prodromus, p. 469.

Definition.—Acantharia with simple spherical porous shell, composed of innumerable very small plates, each of which is pierced by one radial porule. Twenty radial spines of equal size meeting in the centre of the shell and disposed according to the Müllerian law of the Icosacantha, sometimes short and enclosed in the shell, at other times long and piercing it (rarely rudimentary or quite absent). Shell pierced therefore either by twenty larger perspinal pores or by eighty smaller aspinal pores. Central capsule spherical, enclosed in the porous shell.

The family Sphærocapsida, founded by me in 1881 for the single genus Sphærocapsa, represents a very peculiar and remarkable group of the Acanthophracta, very different from the five other families of this suborder, and probably derived, independently of them, directly from the Acanthonida. Whilst the lattice-shell of the five other families is composed of the meeting branches of lateral apophyses of the twenty spines, and its meshes are all or partly the intervals between these apophyses, in the Sphærocapsida the spherical shell has quite another structure, and is composed of innumerable small plates (each with one pore) which are secreted on the surface of the spherical calymma, independently of the twenty radial spines, which do not possess true apophyses.

In all Sphærocapsida the structure of the spherical shell is quite peculiar and different from that of all other Radiolaria. It is composed everywhere of innumerable very small plates or aglets, which are connected irregularly like paving-stones, and form a single continuous layer or pavement on the surface of the spherical calymma (Pl. 133, fig. 11, a; Pl. 135, figs. 8, 10). The small plates or paving-stones, which we will call "aglets," are connected at their meeting edges by a kind of cement, and form together with it a continuous thick capsule of acanthin. The form of the aglets is commonly more or less irregular, roundish or polygonal, sometimes longish (Pl. 133, fig. 11, a), more rarely it becomes rather regular, hexagonal, square, or roundish (Pl. 135, fig. 8). Usually all aglets of one and the same individual are of nearly equal size, between 0.01 and 0.02 in diameter, rarely less or more. The outer face of the aglets is more or less concave, so that the elevated meeting edges of the neighbouring aglets commonly form together a prominent network of crests (Pl. 135, figs. 8, 10); rarely the meeting edges partly cover one another like squamules (Pl. 133, fig. 11, a). Each small plate or aglet is pierced in its centre by a single radial canalicule or porule. The dimpled surface, so produced, resembles somewhat the dimpled plates of Ceriaspis, &c. Different from these innumerable very small dimples of the surface are the twenty larger "spinal dimples," or the concave larger plates, which are originally pierced by the twenty radial spines. Before we describe these, we must examine the spines themselves.

The twenty radial spines of all observed Sphærocapsida (sixteen species) agree perfectly with those of the genus Acanthonia (p. 749), and especially with Acanthonia tetracopa, Acanthonia denticulata, &c. All twenty spines, regularly disposed according to the Müllerian law of the Icosacantha, are of equal size, constantly four-edged prismatic, of equal breadth throughout their whole length. The prominent four edges are parallel, sometimes smooth, at other times elegantly denticulated. The central bases of the twenty spines are pyramidal, without leaf-cross, and propped one upon another with their triangular faces, as in the majority of the Acanthonida.

The relation of the twenty radial spines to the spherical shell exhibits in the five genera described very peculiar and important differences. In the first described genus, in Sphærocapsa, the spines are exactly as long as the shell-radius, and therefore are not prominent over the surface of the shell, with which they are firmly connected; the truncated distal end of the spine lies therefore here in the surface of the shell itself, and is connected with it by its four edges, between which four open aspinal pores remain, as in Tessaraspis, &c. (Pl. 135, figs. 6-10). In the next allied genus, Astrocapsa (Pl. 133, figs. 9, 10), the spines are longer than the shell-radius, and therefore more or less prominent over its surface; the piercing part of each spine is also surrounded by four aspinal pores. In the two following genera, Porocapsa and Cannocapsa (Pl. 133, figs. 7, 8), the radial spines are shorter than the shell-radius and therefore quite hidden and withdrawn inside the shell, which they do not reach. But in the ideal prolongation of each spine the shell is pierced by a single large opening, the "perspinal pore" or "perspinal hole," composed of the four united aspinal pores. Whilst in Porocapsa the perspinal pores are simple, they are prolonged in Cannocapsa into cylindrical tubes, open at both ends. The twenty perspinal holes of these Porocapsida are therefore derived by confluence of the eighty original aspinal pores of the Astrocapsida and preserve the same regular disposition, according to the Müllerian law of the Icosacantha. Finally, the same law as is valid also in the last genus is found in Cenocapsa; here the radial spines have completely disappeared, and the whole skeleton is a simple sphere, but of the same structure, and with the same twenty perspinal pores as in Porocapsa. It is very interesting that this spineless Cenocapsa among the Acantharia exhibits the same shell (a simple hollow sphere) as a last reduced form, which Cenosphæra among the Sphærellaria produces as a primitive ancestral form of numerous genera.

The Central Capsule of the Sphærocapsida is spherical, constantly smaller than the enclosing concentric shell, and separated from it by the calymma. Its structure seems to be the same as in the Acanthonida, and specially in the Astrolonchida. The pseudopodia (not yet observed) are probably protruded only through the twenty perspinal holes or the eighty aspinal pores.

Synopsis of the Genera of Sphærocapsida.

Subfamily 1. Astrocapsida, Haeckel.

Definition.—Radial spines connected with the porous shell, as long as or longer than its radius. Therefore the shell pierced by eighty aspinal pores (four around each spine).

Genus 344. Sphærocapsa,^[384] Haeckel, 1881, Prodromus, p. 469.

Definition.—Sphærocapsida with twenty radial spines as long as the radius of the shell, without external prolongation; therefore their distal ends inserted in the perspinal holes, each of which is composed of four aspinal pores.

The genus Sphærocapsa is the most common form of the Sphærocapsida, and comprises those species in which the radial spines are as long as the radius of the shell, and therefore are connected with the margin of its aspinal holes, but not prolonged beyond its surface.

1. Sphærocapsa cruciata, n. sp. (Pl. 135, figs. 6, 7).

Aspinal holes nearly circular, with flat radially striated margin. Four aspinal pores of each hole rounded equilateral triangular. Porules of the shell simple, without ring and dimple. Four edges of the spines smooth.

Dimensions.—Diameter of the shell 0.4 to 0.5, of the central capsule 0.3 to 0.4.

Habitat.—North Atlantic, Færöe Channel (Gulf Stream), 1880, John Murray, surface.

2. Sphærocapsa dentata, n. sp. (Pl. 135, fig. 9).

Aspinal holes four-lobed, with broad concave margin denticulated on the periphery. Four aspinal pores of each hole pear-shaped, oblong, elevated in the centre. Porules of the shell simple, without ring and dimple. Four edges of the spines denticulate.

Dimensions.—Diameter of the shell 0.3 to 0.4, of the central capsule 0.2 to 0.3.

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

3. Sphærocapsa quadrata, n. sp. (Pl. 135, fig. 8).

Aspinal holes square, with concave umbilicus in the centre, surrounded by a convex denticulated margin. Four aspinal pores of each hole nearly square. Shell-porules with a polygonal elevated smooth ring, in the bottom of a shallow dimple. Four edges of the spines smooth.

Dimensions.—Diameter of the shell 0.22, of the central capsule 0.17.

Habitat.—Indian Ocean, south of Australia, Station 159, surface.

4. Sphærocapsa pavimentata, n. sp. (Pl. 135, fig. 10).

Aspinal holes four-lobed, with broad concave, irregularly crenated and figured margin. Four aspinal pores of each hole violin-shaped. Porules of the shell surrounded by an irregularly oblong ring with thick elevated, elegantly crenated margin. Four edges of the spines smooth.

Dimensions.—Diameter of the shell 0.36, of the central capsule 0.3.

Habitat.—South-east Pacific (off Valparaiso), Station 298, surface.

Genus 345. Astrocapsa,^[385] n. gen.

Definition.—Sphærocapsida with twenty radial spines longer than the radius of the shell, piercing its perspinal holes, with free external prolongation; therefore with four aspinal pores around each spine.

The genus Astrocapsa differs from the preceding Sphærocapsa in the external prolongation of the radial spines piercing the perspinal holes; it assumes therefore the common shape of the Dorataspida more than the other Sphærocapsida do.

1. Astrocapsa tritonis, n. sp.

Aspinal holes circular, with smooth convex margin and four circular aspinal pores. Porules of the shell simple, without ring and dimple. Four edges of the spines smooth, their outer free part about as long as the diameter of the shell.

Dimensions.—Diameter of the shell 0.25; outer length of the spines 0.3.

Habitat.—North Atlantic, Færöe Channel, Gulf Stream (expedition of H.M.S "Triton," August, 1882), John Murray, surface.

2. Astrocapsa stellata, n. sp. (Pl. 133, fig. 10).

Aspinal holes cruciform, with high crenated margin. Four aspinal pores of each hole egg-shaped. Porules of the shell simple, without ring and dimple. Four edges of the spines denticulate; their outer free part twice to three times as long as the diameter of the shell.

Dimensions.—Diameter of the shell 0.36; outer length of the spines 0.5 to 0.8.

Habitat.—Antarctic Ocean (near Kerguelen), Station 152, surface.

3. Astrocapsa quadrifida, n. sp.

Aspinal holes four-lobed, with four prominent teeth between the four roundish aspinal pores. Porules of the shell surrounded by a high polygonal smooth ring. Four edges of the spines denticulate; their outer free part about as long as the radius of the shell.

Dimensions.—Diameter of the shell 0.28; outer length of the spines 0.15.

Habitat.—North Pacific, Station 241, surface.

4. Astrocapsa coronata, n. sp. (Pl. 133, fig. 9).

Aspinal holes circular, with a coronet of numerous thin parallel teeth. Porules of the shell in dimples on irregular polygonal small plates, with coronated ring. Four edges of the spines denticulate; their outer free part about twice as long as the diameter of the shell.

Dimensions.—Diameter of the shell 0.4 to 0.5 ; outer length of the spines 1.0 to 1.2.

Habitat.—North Atlantic, Færöe Channel, Gulf Stream (expedition of the "Knight Errant," 1880), John Murray, surface and at depths varying from 10 to 200 fathoms.

Subfamily 2. Porocapsida, Haeckel.

Definition.—Radial spines not connected with the porous shell, shorter than its radius; therefore the shell pierced by twenty perspinal pores (each one in the ideal radial prolongation of one spine).

Genus 346. Porocapsa,^[386] n. gen.

Definition.—Sphærocapsida with twenty radial spines shorter than the radius of the shell; therefore their distal ends not connected with the twenty perspinal holes, which are simple, not prolonged into radial tubes.

The genus Porocapsa and the following Cannocapsa form together the small sub-family of Porocapsida, distinguished by the peculiar reduction or retrograde development of the twenty radial spines; all these twenty are present and disposed according to the Müllerian law of the Icosacantha, but they are shorter than the radius of the shell and therefore do not reach it. In the ideal prolongation of the spines the shell is pierced by twenty simple quadrangular or circular perspinal holes.

1. Porocapsa murrayana, n. sp. (Pl. 133, fig. 7).

Perspinal holes cruciform, with smooth thickened margin or with four short teeth between the four lobes. Porules of the shell simple, without ring and dimple. Four edges of the spines smooth. (Differs from Sphærocapsa cruciata, Pl. 135, figs. 6, 7, living in the same locality, mainly in the reduction of the radial spines, which do not reach the shell.)

Dimensions.—Diameter of the shell 0.28; length of the spines 0.2.

Habitat.—North Atlantic, Færöe Channel, Gulf Stream (expedition of H.M.S. "Triton," August 1882), John Murray, surface and in depths from 40 to 640 fathoms.

2. Porocapsa tetrodon, n. sp.

Perspinal holes cruciform, with four triangular prominent teeth between the four lobes of the cross. Porules of the shell with an elevated polygonal ring, in the bottom of a dimple. Four edges of the spines elegantly denticulate.

Dimensions.—Diameter of the shell 0.36; length of the spines 0.12.

Habitat.—North Pacific, Station 253, surface.

3. Porocapsa octodon, n. sp.

Perspinal holes square, with four larger prominent teeth on the sides of the square, and four smaller teeth on its corners. Porules of the shell surrounded by an elevated ring with elegantly crenated irregular margin. Four edges of the spines smooth.

Dimensions.—Diameter of the shell 0.48; length of the spines 0.18.

Habitat.—Arctic Ocean (Greenland), in the contents of the stomach of the Peromedusa, Periphylla hyacinthina.

4. Porocapsa coronodon, n. sp.

Perspinal holes circular, with ciliated margin, which forms a crown of sixteen to twenty-four thin parallel teeth. Poruli of the shell surrounded by an elevated ring with high crenated margin. Four edges of the spines denticulate.

Dimensions.—Diameter of the shell 0.55; length of the spines 0.15.

Habitat.—Antarctic Ocean, Station 154, surface.

Genus 347. Cannocapsa,^[387] n. gen.

Definition.—Sphærocapsida with twenty radial spines shorter than the radius of the shell; therefore their distal ends not connected with the twenty perspinal holes, which are prolonged outside into radial tubes (each one in the radial ideal prolongation of one inner spine).

The genus Cannocapsa, exhibits the same peculiar reduction of the radial spines as the foregoing Porocapsa; the spines are also here shorter than the shell-radius and do not therefore reach the perspinal holes of the shell. But whilst these latter are simple in Porocapsa, they are prolonged into radial tubules in Cannocapsa; the outer surface bears therefore twenty such cylindrical tubules, separated by a short distance from the inner enclosed spines, but disposed quite regularly according to the law of Icosacantha.

1. Cannocapsa osculata, n. sp.

Perspinal holes prolonged into short cylindrical tubuli, the length of which about equals their diameter. Both ends of the tubuli with smooth thickened margin. Poruli of the shell simple, without ring and dimple. Four edges of the spines smooth; their length equals about four-fifths of the shell-radius.

Dimensions.—Diameter of the shell 0.24; length of the spines 0.09, of the tubuli 0.01.

Habitat.—North Atlantic, Færöe Channel (Gulf Stream), surface, John Murray.

2. Cannocapsa stethoscopium, n. sp. (Pl. 133, fig. 8).

Perspinal holes prolonged into cylindrical tubuli, half as long as the shell radius. Both ends of the tubuli with smooth thin trumpet-shaped margins. Poruli of the shell simple, without ring and dimple. Four edges of the spines smooth; their length about equals three-fourths of the shell-radius.

Dimensions.—Diameter of the shell 0.2; length of the spines 0.08, of the tubuli 0.05.

Habitat.—South Atlantic (west of Tristan da Cunha), Station 333, surface.

3. Cannocapsa tubulosa, n. sp.

Perspinal holes prolonged into cylindrical tubuli, which are as long as or longer then the shell-radius. Both ends of the tubuli with smooth thickened margin. Poruli of the shell simple, without ring and dimple. Four edges of the spines smooth; their length scarcely equals half the shell-radius.

Dimensions.—Diameter of the shell 0.15; length of the spines 0.03, of the tubuli 0.08.

Habitat.—South Atlantic (east coast of Patagonia), Station 318, surface.

Subfamily 3. Cenocapsida, Haeckel.

Definition.—Radial spines completely reduced and absent; shell cavity therefore simple; shell pierced by twenty perspinal pores (each placed in the direction of one radial spine which has disappeared).

Genus 348. Cenocapsa,^[388] n. gen.

Definition.—Sphærocapsida without radial spines, with simple cavity of the spherical shell, which is pierced by twenty perspinal holes (each one placed in the radial direction of one spine which has disappeared).

The genus Cenocapsa comprises only a single species, but is very remarkable in that it is the most reduced form among all Sphærocapsida. The twenty radial spines of the Icosacantha have perfectly disappeared by complete retrograde metamorphosis, and the only evidence of their former existence (in the ancestral genus Porocapsa) are the twenty perspinal holes remaining in the shell. Cenocapsa is the only form of Acantharia which possesses no radial spines.

1. Cenocapsa nirvana, n. sp. (133, fig. 11, 11a, 11c).

Perspinal holes four-lobed, cruciform, with four short triangular teeth between the four lobes of the cross. Poruli of the shell in the bottom of an elliptical dimple surrounded by an elevated ring.

Dimensions.—Diameter of the shell 0.2 to 0.3, of the aspinal holes 0.02.

Habitat.—North Pacific, Station 248, surface.

Family XL. Dorataspida, Haeckel (Pls. 134-138).

Dorataspida, Haeckel, 1862, Monogr. d. Radiol., p. 412.

Definition.—Acantharia with simple spherical lattice-shell, composed of the branched apophyses of twenty equal radial spines meeting in its centre and disposed according to the Müllerian law of Icosacantha. Central capsule spherical, enclosed in the fenestrated shell.

The family Dorataspida is the most important family of the Acanthophracta, or of those Acantharia in which the radial spines are connected by a complete extracapsular lattice-shell. The Dorataspida represent probably the ancestral stock of this whole order, with the exception of the Sphærocapsida. The four following families of the order may be easily derived from the Dorataspida. The number of genera (seventeen) and of species (one hundred and eight) is in this family greater than in the other five families together. When I constituted that family in my Monograph 1862, it comprised only one genus, Dorataspis, with seven species. The nearly allied genus Haliommatidium (Phatnaspis) belongs to the Belonaspida.

The Dorataspida differ from the other Acanthophracta in the simple spherical lattice-shell, which is composed of the meeting apophyses of the twenty radial spines. In three other families of the suborder the shell is not spherical, but ellipsoidal (Belonaspida), discoidal (Hexalaspida), or diploconical (Diploconida). In the Phractopeltida the spherical shell is double, composed of two concentric lattice-spheres. In the Sphærocapsida the simple spherical shell is not composed of the apophyses of the spines, but of innumerable small plates.

The family Dorataspida may be divided into two very different subfamilies, which are probably derived, independently of one another, from two different subfamilies of the Astrolonchida. The first subfamily, Diporaspida, exhibits on each radial spine two opposite apophyses, like its ancestral group, the Phractacanthida (p. 753); whereas the second subfamily, Tessaraspida, possesses on each radial spine four crossed apophyses (opposite in pairs), like its ancestral group, the Stauracanthida (p. 758). Therefore the composition of the spherical shell, produced by the meeting branches of the tangential apophyses, is essentially different in the two subfamilies: in the Diporaspida each radial spine is surrounded by two opposite primary aspinal meshes, in the Tessaraspida by four crossed primary aspinal meshes.

Another principle of division may be established for the whole family by the different mode of composition of the shell, and regarding this important difference we may distinguish also two different subfamilies as Cladophracta and Peltophracta. In the first and simpler subfamily, the Cladophracta, the shell is composed totally (or sometimes partially) of the meeting branches of the apophyses of the neighbouring spines; but in each single spine (or in the most part of them) the branches of the apophyses are not united, and form no lattice-plate (Pl. 137, figs. 1 to 8). Whereas in the Peltophracta the shell is composed constantly of twenty perforated plates, as in each single spine the branches of its apophyses are united and form a fenestrated shield with two or four (and sometimes numerous) pores (Pls. 135, 136, 138).

In the Diporaspida as well as in the Tessaraspida we find numerous representatives of the two groups of the Cladophracta and of the Peltophracta; therefore the whole family of Dorataspida may be divided into four different tribes. The Diporaspida (with two opposite apophyses on each spine) are partly Cladophracta (the Phractaspida, Pl. 137, figs. 1-4), partly Peltophracta (the Ceriaspida, Pl. 138). On the other hand the Tessaraspida (with four crossed apophyses on each spine) are also partly Cladophracta (the Stauraspida, Pl. 137, figs. 5-8), partly Peltophracta (the Lychnaspida, Pls. 135-136). The differences and relations of these tribes are placed synoptically in the following table:—

All Dorataspida are true Icosacantha, and the twenty spines, composing the spherical shell, are equally developed, regularly disposed according to the Müllerian law, and of equal size; also the distance of their plates from the common centre is equal. Nevertheless they are never of perfectly the same form; in consequence of their peculiar disposition in five zones (each with four spines) certain slight differences are effected, so that with accurate knowledge of the peculiar shell-composition it is generally not difficult to distinguish the spines of the equatorial, the two tropical, and the two polar zones.

Already the central bases, by which the twenty spines are united in the centre of the sphere, exhibit certain differences in the five zones. Commonly these bases are small pyramids, all meeting with their apex in the centre, and the triangular faces of the neighbouring pyramids are supported one upon another. The four equatorial pyramids are commonly six-sided, the other sixteen five-sided; but sometimes there are eight six-sided and twelve five-sided basal pyramids; two opposite polar spines on each pole having a six-sided base (like the four equatorial), the other two polar spines on each pole having a five-sided base (like the eight tropical). Rarely the central bases are perfectly grown together, forming a single spherical central piece of acanthin.

The three different fundamental forms of radial spines, which are found in all Acantharia, the cylindrical, the two-edged, and the four-edged (spines with circular, with elliptical, and with square transverse section respectively) occur also in the different groups of Dorataspida; but commonly the two-edged or compressed form is prevalent in the Diporaspida, the four-edged or quadrangular form in the Tessaraspida. In the majority of species the spines are thickened in the shell-face, where the apophyses arise, and thinner towards the two ends. Usually the outer or distal part of the spine (outside the shell) is longer than the inner or proximal part (inside the shell). The distal apex is commonly simple, conical or pyramidal, rarely bifid or truncate. The edges of the spines are commonly smooth, rarely denticulate or serrate.

The apophyses, or the lateral transverse processes of the radial spines, in the Dorataspida assume the greatest variety and complexity in form, size, mode of ramification, and in composition of the shell. An expert and practised observer may determine easily the range of each spine, whether it be an equatorial (c), or a tropical (b, d), or a polar spine (a, e, Pls. 133-138). The two opposite apophyses of the Diporaspida, as well as the four crossed apophyses of the Tessaraspida, lie constantly in certain meridian planes of the spine, which have a legitimate signification for each of the five zones. The comparative morphology of this regular disposition of the apophyses and the regular meeting of their branches is of the greatest interest, and necessary for the complete understanding of the complicated structure of these wonderful shells.

The pores or meshes of the spherical shell, offering the most varied forms, may generally be divided into two different groups, into sutural and parmal meshes. The sutural pores are bordered by the meeting branches of the apophyses of two, three, or four neighbouring spines, and therefore also by the sutures in which they meet. The parmal pores on the other hand are bordered only by the united branches of the apophyses of a single spine and pierce the shield or lattice-plate formed by them. Therefore the shell-meshes of the Cladophracta are all sutural pores (Pl. 137, figs. 1-8; rarely and only in a part of the spines parmal pores also: Zonaspis, Dodecaspis); whereas the shell-meshes of the Peltophracta, piercing the shields or lattice-plates of all twenty spines, are always partly sutural, partly parmal pores (Pls. 135, 136, 138). The parmal pores again may be divided into two different groups—aspinal and coronal pores. Aspinal pores ("ad spinam") are those which lie immediately on the sides of the radial spine and are bordered by the primary branches of its apophyses; therefore constantly only two in the Diporaspida, four in the Tessaraspida. Coronal pores on the contrary are those which lie in the periphery of the lattice-plates, surrounding in a circle or crown the aspinal pores and not touching the spine itself. In Dorataspis, Ceriaspis, Tessaraspis, Lychnaspis, &c., all parmal meshes are only aspinal pores (Pl. 135, figs. 2-5; Pl. 136); whilst in Coscinaspis, Acontaspis, Icosaspis, Hylaspis, &c., one part of the parmal pores is aspinal, one part coronal (Pl. 136). The number, form, and size of the coronal pores is very variable and often very large (sometimes more than a hundred in one plate).

The Cladophracta exhibit a comparatively simple shell-formation; either all twenty spines or at least a part of them not forming lattice-plates. The most primitive form among these is Phractaspis (Pl. 137, figs. 1, 2). The forty apophyses of its twenty spines are simply forked, and their eighty fork-branches united by forty sutures, enclosing twenty-two sutural meshes: two square polar meshes (between the four polar spines on the poles of the spineless axis, a a a a and e e e e); eight triangular circumpolar meshes (each between two polar and one tropical spine, a b a and e d e); eight quadrangular tropical meshes (each between one polar, one equatorial, and two tropical spines, a b c b and e d c d); and four rhomboidal equatorial meshes (between two tropical and two equatorial spines, c b c d). If the fork-branches be again forked (Phractaspidium, Pl. 137, fig. 3), the number of the sutures and sutural meshes is doubled, and the same is the case in Stauraspis, the most simple form of the Tessaraspida.

A peculiar small group, and an interesting transition for the Cladophracta to the Peltophracta, is presented by the Zonaspida among the Tessaraspida (Zonaspis and Dodecaspis). Here only one part of the radial spines bears lattice-plates, the other part not. In Zonaspis the four equatorial spines bear lattice-plates, the sixteen other only free branches of the apophyses. In Dodecaspis twelve spines are provided with lattice-plates (four equatorial and eight polar spines), whilst the eight other (tropical) spines are devoid of them.

The Peltophracta exhibit a great variety in the form and composition of their twenty lattice-plates or fenestrated shields. In the most simple case (a part of Dorataspis and Diporaspis) the shell is composed of four (equatorial) hexagonal plates, and sixteen pentagonal plates (four tropical and four polar); in this case the four polar plates meet on each pole in one common point. More commonly, however, the shell seems to be composed of eight hexagonal plates (four equatorial and the four polar spines of the hydrotomical plane) and twelve pentagonal plates (eight tropical and the four polar spines of the geotomical plane); in this case only two (hexagonal) polar plates meet on each pole in a suture which separates the two other (pentagonal) polar plates (Pl. 138, fig. 4). In the majority of the Dorataspida the composition of the shell is much more complicated and often very difficult to understand. Often the surface of the plates is covered with a network of elevated crests, by which concave blind dimples are separated (Ceriaspis, Hystrichaspis, Pl. 138); and sometimes these dimples become pierced by coronal pores (Pl. 138, fig. 11, &c.).

Peculiar by-spines or "accessory spines" cover the outer surface of the shell in a great part of Dorataspida, and commonly these most characteristic by-spines are not placed radially, but parallel to the radial spine, from the lattice-plate of which they arise (Pl. 135, figs. 1, 5; Pl. 137, figs. 4-8; Monogr. d. Radiol., 1862, Taf. xxi. figs. 8, 9). They are commonly placed perpendicular to the sutural condyles, or the branch-ends of the apophyses; so that close to each suture arises a pair of divergent by-spines, belonging to the meeting apophyses of the two neighbouring spines, which meet in the suture (Pl. 137, fig. 4). Rarely these thin, bristle-shaped by-spines are quite simple and straight, commonly they undulate or are zigzag and often armed with recurved hooks. Sometimes they are also forked or arborescent (Pl. 138, fig. 7).

The Central Capsule of the Dorataspida is constantly spherical and about one-third smaller than the enclosing shell, from which it is separated by the spherical calymma. The membrane of the central capsule is commonly rather thin, and pierced by the twenty radial spines, meeting in the centre of the capsule. Between these lie innumerable small pores for the radiating pseudopodia; however, in many cases (and perhaps constantly) these pores exhibit a certain regular disposition. In many species the central capsule encloses Xanthellæ or symbiotic yellow unicellular Algæ. The nucleus becomes cleft very early in the majority of Dorataspida.

Synopsis of the Genera of Dorataspida.