The genus Amphipyle opens the large series of Diplozonaria, comprising all Pylonida, the shell of which is composed of two concentric systems of latticed girdles; the first system constituting the characteristic "trizonal medullary shell" or "Larnacilla-shell;" the second system composed of one to three girdles of the second order. The first system lies inside, the second outside the central capsule. In Amphipyle only the first (transverse) girdle of the second system becomes developed, and therefore on both poles of the principal axis are two large open gates. Amphipyle repeats the two-winged form of Monozonium; but whilst the medullary shell in this latter is a simple central chamber, it is here a trizonal Larnacilla-shell.
Subgenus 1. Amphipylissa, Haeckel.
Definition.—Cortical shell smooth or thorny, but without large, symmetrically disposed spines.
1. Amphipyle aceros, n. sp.
Cortical shell quite smooth, without any spines or thorns. Both lateral wings (or opposite half girdles) semilunar, with convex lateral crest, three times as long as broad; twice as broad as the trizonal lentelliptical medullary shell.
Dimensions.—Length of the medullary shell 0.05, breadth 0.03; length of each lateral wing (or principal dimension of the latticed girdle) 0.18, breadth of it (or transverse dimension of each girdle-tube) 0.06.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
2. Amphipyle stenoptera, n. sp.
Cortical shell smooth, without spines. Both lateral wings of the same breadth as the lentelliptical medullary shell, but five times as long as broad, prolonged on both ends into cylindrical latticed tubes, with rectilinear lateral crest.
Dimensions.—Length of the medullary shell 0.06, breadth 0.04; length of each cylindrical lateral wing 0.2, breadth 0.04.