Definition.—Coccodiscida with four chambered arms on the margin of the circular or quadrangular disk, crossed in two equatorial diameters, without a connecting patagium.
The genus Astractura has the form of a regular cross, four radial arms being opposite in two equatorial diameters perpendicular one to another. In the Porodiscida the same form is repeated by Stauralastrum, in the Spongodiscida by Spongasteriscus. The oldest known species of the genus is Astromma aristotelis of Ehrenberg, in which genus this author confounded triradial and four-radial forms.
Subgenus 1. Astracturium, Haeckel.
Definition.—Distal end of the arms blunt or truncated, without terminal spines.
1. Astractura ordinata, n. sp.
? Astromma sp., Bury, 1862, Polycystins of Barbados, pl. xiv. fig. 3.
Phacoid shell three times as broad as the medullary shell, with six pores on its radius, without a completely developed chambered ring. Arms trapezoidal, somewhat longer than the radius of the disk, at the truncated distal end as broad as long, at the base one-third smaller.
Dimensions.—Diameter of the phacoid shell 0.08, of the medullary shell 0.03; length of the arms 0.05, distal breadth 0.05, basal breadth 0.035.
Habitat.—Tropical Atlantic, Station 348, depth 2450 fathoms; also fossil in Barbados.
2. Astractura clavigera, n. sp.