The genus Pylospira follows after Larcospira as the second genus of Larcospirida; but in this latter the spiral of the cortical shell is formed by the transverse girdle (or the first lattice-girdle of the Diplozonaria), whilst in Pylospira it is produced by the lateral girdle, or the second lattice-girdle of that group. Therefore Pylospira may be derived phylogenetically from Tetrapyle in the same manner as Larcospira from Amphipyle. Whilst in this latter the first cause of the spiral turning, the unequal growth of both girdle-wings, proceeds from the transverse girdle, in Pylospira it proceeds from the lateral girdle. One of its wings overgrows the other, turning around the sagittal axis. If the second wing do not become developed, the spiral remains simple and represents the subgenus Pylospirema; but if afterwards the second wing follow the example of the first and overgrow it from the other side, we reach the typical form of the second subgenus, Pylospironium, with a double spiral.
Subgenus 1. Pylospirema, Haeckel.
Definition.—Spiral convolutions or turns of the cortical shell simple, only one single wing of the lateral girdle turning around the sagittal axis.
1. Pylospira tetrapyle, n. sp.
Cortical shell with thorny surface, five times as long as the lentelliptical medullary shell. Perimeter of the lateral plane elliptical, one and a half times as long as broad. Four internal gates (as in Tetrapyle, between the complete lateral wings of the transverse girdle) kidney-shaped. One principal wing of the lateral girdle turns around the other in one and a half to two simple spiral turns.
Dimensions.—Length of the cortical shell 0.21, breadth 0.15; length of the medullary shell 0.04, breadth 0.03.
Habitat.—Pacific, central area, Station 268, depth 2900 fathoms.
2. Pylospira octopyle, n. sp. (Pl. [49], fig. 4).
Cortical shell with thorny surface, three times as long as the lentelliptical medullary shell. Perimeter of the lateral plane elliptical or nearly hexagonal, with four rounded corners (on the poles of two crossed diagonal axes), one and a third times as long as broad. Eight internal egg-shaped gates (as in Octopyle), between the complete lateral wings of the transverse girdle, and two axial beams in the principal axis. One single wing of the lateral girdle turns around the other in two to three simple spiral turns.
Dimensions.—Length of the cortical shell 0.18, breadth 0.14; length of the medullary shell 0.06, breadth 0.035.