Phacoid shell twice as broad as the medullary shell, with four to five pores on its radius. Arms slender, lanceolate, three times as long as the diameter of the phacoid shell and five times as long as broad in the middle part; distal ends blunt. Patagium enveloping only the basal half of the arms, forming a regular triangle with concave sides.
Dimensions.—Diameter of the phacoid shell 0.08, of the medullary shell 0.035; length of the arms 0.2, greatest breadth 0.04.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms; fossil in the rocks of Barbados.
4. Hymenactura hexagona.
Hymeniastrum sp., Bury, 1862, Polycystins of Barbados, pl. xv. fig. 3.
Phacoid shell twice as broad as the medullary shell, with four to five pores on its radius. Arms nearly equilateral triangular, three times as long as the diameter of the phacoid shell, at the truncated distal end nearly three-fourths as broad as long, at the narrow base only one-fourth as broad. Patagium complete, enveloping the arms completely, so that the whole body represents a regular hexagonal disk.
Dimensions.—Diameter of the phacoid shell 0.08, of the medullary shell 0.04; length of the arms 0.18, basal breadth 0.04, distal breadth 0.16.
Habitat.—Fossil in the rocks of Barbados and Nicobar.
Subgenus 2. Hymenactinium, Haeckel.
Definition.—Distal ends of the arms armed with terminal spines.