Definition.—Astrolonchida with four apophyses on each radial spine, opposite in pairs in cross form, and forming a lattice-plate by communicating branches.
The genus Phatnacantha has been derived from the preceding Stauracantha by concrescence of the branches of the apophyses. Therefore each spine bears a lattice-plate or a fenestrated shield. If the growing plates of the neighbouring spines meet together, then the characteristic lattice-shell of the Tessaraspida is perfect. Either each plate bears four crossed pores (like Tessaraspis), or a larger number of pores (four aspinal and four to eight or more coronal) like Icosaspis.
1. Phatnacantha tessaraspis, n. sp.
Spines quadrangular prismatic, with simple pyramidal apex and small basal leaf-cross. Each spine bears in the basal half a square plate, which is perforated by four square pores; margin of the plate with twelve short straight teeth.
Dimensions.—Length of the spines 0.12, breadth 0.005.
Habitat.—Central Pacific, Station 265, surface.
2. Phatnacantha icosaspis, n. sp. (Pl. [130], fig. 6).
Spines quadrangular with prominent edges; prismatic in the basal half, with a broad basal leaf-cross, pyramidal in the distal half, with a simple or truncate apex. Each spine bears in the middle part a square plate, which is perforated by eight to sixteen or more square pores (four aspinal and four to eight or more coronal); margin of the plate with twenty-four to forty-eight or more short straight irregular compressed teeth.
Dimensions.—Length of the spines 0.18, breadth 0.008.
Habitat.—Central Pacific, Station 273, surface.