B. Tribe II. Lychnaspida, Haeckel, 1881, Prodromus, p. 467.

Definition.—Dorataspida with twenty perforated plates or fenestrated shields (each plate at least with four pores), produced by union of the branches of the four crossed apophyses, which arise, opposite in pairs, from each radial spine. The spherical shell is composed of the twenty plates united by sutures (rarely by concrescence).

Genus 362. Tessaraspis,[[402]] Haeckel, 1881, Prodromus, p. 468.

Definition.—Dorataspida with twenty plates, which are perforated by eighty aspinal pores (four crossed pores in each plate). Surface smooth, without by-spines.

The genus Tessaraspis introduces the series of Lychnaspida, which comprise all those Dorataspida in which the shell is composed of twenty plates, each of which is perforated by four primary aspinal pores. In Tessaraspis and Lychnaspis each plate exhibits only these four primary pores, whilst in Icosaspis and Hylaspis they become surrounded by a circle of secondary or coronal pores. If in Stauraspis, the common ancestral form of the Tessaraspida, the four crossed apophyses of each single radial spine became recurved and united together, we should have the typical plate of Tessaraspis, in which the piercing radial spine is surrounded by four crossed pores of equal size. The number of sutural pores, between the neighbouring plates, is variable; usually each plate is surrounded by a circle of eight to twelve sutural pores. The sutures between the meeting condyles of the apophyses usually remain open; but in some species they become obliterated (subgenus Tessaraspidium).

Subgenus 1. Tessarasparium, Haeckel.

Definition.—Condyles of the neighbouring plates connected by permanent open sutures; therefore the whole shell is composed of twenty separated pieces of acanthin.

1. Tessaraspis arachnoides, n. sp. (Pl. [136], fig. 1).

Parmal pores pentagonal, ten to twenty times as broad as the thin thread-like bars, on an average of about the same size as the irregular sutural meshes; the majority of the latter are either triangular or hexagonal. Radial spines very thin and long, cylindrical, their outer part two to four times as long as the inner. As the insertion of the spines is on the highest point of the plates, the shell becomes polyhedral (dodecahedral?).

Dimensions.—Diameter of the shell 0.15 to 0.17, of the parmal meshes 0.02 to 0.025, sutural meshes 0.01 to 0.03, bars 0.002.