Parmal meshes twelve hundred to sixteen hundred; in the centre of each plate a cross of four large, somewhat oblong, octahedral aspinal pores, between these four rhombic smaller angular pores; around this rosette an inner circle of twelve to sixteen larger and an outer circle of fifty to sixty very small coronal pores; the latter smaller than the sutural pores. On each condyle one bristle-shaped zigzag by-spine, with recurved thin hooks, half as long as the radius. Twenty radial spines, very long, cylindrical, smooth.
Dimensions.—Diameter of the shell 0.25 to 0.3, aspinal pores 0.022, other pores 0.002 to 0.015, bars 0.002.
Habitat.—South Pacific, Station 295, surface.
Family XLI. Phractopeltida, Haeckel (Pl. [133], figs. 1-6).
Phractopeltida, Haeckel, 1881, Prodromus, p. 468.
Definition.—Acantharia with double spherical lattice-shell, composed of the branched apophyses of twenty radial spines meeting in its centre, and disposed according to the Müllerian law of Icosacantha. Central capsule spherical, enclosing the inner and surrounded by the outer concentric shell.
The family Phractopeltida differs from all other Acantharia in the development of a double spherical shell, composed of two concentric lattice-spheres, which are united by twenty radial spines meeting in the common centre. We could therefore oppose the Phractopeltida as Diplophracta to all other Acanthophracta as Haplophracta (with simple shell). The former exhibit a relation to the latter, similar to that exhibited by the Dyosphærida to the simple Monosphærida among the Sphæroidea.
In my Monograph (1862, p. 423) I described only one genus appertaining to this family, Aspidomma. I founded it upon the singular Phractopelta, described by J. Müller as Haliomma hystrix. A second species of Aspidomma, the Acanthometra mucronata of J. Müller, was probably an Astrolonche. At that time I placed Aspidomma among the Haliommatida, led by the erroneous opinion that it might represent a transition-form between Dorataspis and Haliomma. But I afterwards gave up this view, as I was convinced that there is no true phylogenetic connection between the acanthinic Dorataspida (Actipylea) and the siliceous Haliommatida (Peripylea). Therefore in my Prodromus (1881, p. 468) I placed Aspidomma among the Dorataspida and changed its name to Phractopelta, to avoid further confusion with the unrelated Ommatida (Sphæroidea). It formed there, with three nearly related genera, the "subfamily Phractopeltida," which we now advance to the higher rank of a separate family. (By a typographical mistake the words are printed in the Prodromus Phractopelma and Phractopelmida, &c., instead of Phractopelta and Phractopeltida, &c.). The detection of other new species appertaining to this family, and a closer anatomical investigation of them, has now led to the distinction of five different genera, characterised by other differences than were employed in 1881 in the provisional system of the "Prodromus."
The two concentric spherical lattice-shells of the Phractopeltida, connected by radial beams, correspond perfectly to those of the double-shelled Dyosphærida (Haliomma, Diplosphæra, &c.), and in both cases we may call the smaller inner the "medullary shell," and the larger outer the "cortical shell." There is no doubt that the double-shelled Phractopeltida must be derived phylogenetically from the simple-shelled Dorataspida (just as we derive the double Dyosphærida from the simple Monosphærida). But it is not yet possible to decide positively which of the two shells is the first formed. Probably the small inner or medullary shell of the Phractopeltida is the first formed, and corresponds to the simple spherical lattice-shell of the Dorataspida; and the larger outer or cortical shell of the former is a later new formation, absent in the latter family. This opinion seems to be confirmed by the genus Orophaspis, the only form among the Dorataspida, in which the radial spines outside the shell bear free latticed apophyses. If these twenty apophyses grow further and meet one another, the second or outer shell of Phractopelta may be formed. But some objections may be raised to this opinion from the peculiar structure and the very small size of the inner shell; and there is some possibility that this latter is a secondary later product inside of the primary cortical shell. The probably phylogenetic series which reveals the origin of the Phractopeltida is the following:—Acanthometron, Zygacantha, Lithophyllium, Phractacantha, Doracantha, Dorataspis, Orophaspis, Phractopelta.
The twenty radial spines exhibit in all Phractopeltida the same characteristic position and relation as in all other Icosacantha, and are constantly arranged according to the Müllerian law in four meridian planes, their distal ends falling into five parallel zones. Their distinction in the majority of the Phractopeltida is not difficult, since the spines of the different zones bear apophyses of different shapes. Sometimes the four equatorial spines are stouter than the sixteen other spines, and often the eight tropical spines are somewhat different in form from the eight polar and from the four equatorial spines. The length of all twenty spines is commonly equal. Their form is usually more or less compressed, two-edged (as in Zygacantha), more rarely cylindrical (as in Acanthometron), or somewhat quadrangular (but not truly prismatic); therefore the transverse section of the spines is commonly elliptical or lanceolate, rarely circular or rhombic, never square; this seems to indicate their origin from Zygacantha. As in all Acantharia, the spines consist of acanthin, not of silex. Their central ends are either perfectly grown together, and form a single star of acanthin, or the triangular faces of their small pyramidal bases are supported one upon another, without true concrescence.