Definition.—Pylodiscida with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers, which are separated by three notches or gates.
Genus 245. Triolena,[[284]] n. gen.
Definition.—Pylodiscida with a simple, spherical or lenticular, central chamber, surrounded by three simple arm-chambers. Notches between the three arms open.
The genus Triolena is the most simple form of all Pylodiscida, and must be regarded as their common ancestral form, from an ontogenetic as well as a phylogenetic point of view. The small shell is composed of a simple, spherical or lenticular, latticed, central chamber, and of three simple, surrounding equal arms, which are also simple latticed chambers, lie in the equatorial plane, and are separated by three equal angles or open gates.
1. Triolena primordialis, n. sp. (Pl. [48], fig. 12).
Arm-chambers trapezoid, nearly square, of the same size as the circular, lenticular, primordial, central chamber. Surface of the disk smooth.
Dimensions.—Diameter of the shell 0.045, of the central chamber 0.015, of each arm 0.015.
Habitat.—Central Pacific, Station 272, depth 2600 fathoms.
2. Triolena tribelone, n. sp.
Arm-chambers lanceolate, of the same breadth as and twice the length of the triangular, central chamber; at the pointed end of each arm is a conical terminal spine (in the equatorial plane). Surface of the disk thorny.