7. Trigonactura trixiphos, n. sp.
Phacoid shell circular, twice as broad as the medullary shell, without a completely surrounding chambered ring, with four pores on its radius. Arms club-shaped, about as long as the diameter of the central disk, at the base half as broad as long, at the rounded distal end broader, and armed with a strong pyramidal terminal spine, nearly as long as the arm itself.
Dimensions.—Diameter of the phacoid shell 0.08, of the medullary shell 0.04; length of the arms (without terminal spine) 0.08, basal breadth 0.04, distal breadth 0.06.
Habitat.—Fossil in the rocks of Barbados.
Genus 207. Hymenactura,[[246]] Haeckel, 1881, Prodromus, p. 459.
Definition.—Coccodiscida with three chambered arms on the margin of the circular or triangular disk, connected by a spongy patagium.
The genus Hymenactura differs from the foregoing Trigonactura in the spongy patagium between the arms, and bears therefore to it the same relation that among the Porodiscida Hymeniastrum does to the simpler Dictyastrum. The oldest known species of this genus is Hymenactura pythagoræ, described by Ehrenberg as Hymeniastrum pythagoræ, but differing from this in the structure of the central disk.
Subgenus 1. Hymenacturium, Haeckel.
Definition.—Distal end of the arms blunt or truncated, without terminal spines.
1. Hymenactura archimedis, n. sp. (Pl. [38], fig. 8).