Page:Scientific results HMS Challenger vol 18 part 1.djvu/852

644 13. Amphipyle callizona, n. sp. (Pl. 9, fig. 6).

Cortical shell smooth, with sixteen strong and short conical spines, lying opposite in pairs in two crossed diagonal planes; each lateral wing four-sided prismatic, its lateral face concave, both ends truncated, and each end provided with four divergent spines. Length of the wings somewhat greater, but breadth smaller, than that of the medullary shell.

Dimensions.—Length of the medullary shell 0.07, breadth 0.05; length of the lateral wings 0.09, breadth 0.03.

Habitat.—Western Tropical Pacific, Station 225, depth 4475 fathoms.

Definition.— with trizonal lentelliptical medullary shell, surrounded by two crossed latticed cortical girdles, one smaller (primary) transverse, and one larger (secondary) lateral girdle. Four gates between the two cortical girdles simple, without a sagittal septum.

The genus Tetrapyle, till 1881 the only known genus of the whole family, was founded by Johannes Müller in 1858, and clearly illustrated by the Mediterranean (and common cosmopolitan) Tetrapyle octacantha, hitherto the best known type of this family. Afterwards (1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 832) Ehrenberg founded the genus Schizomma for a nearly allied form, which exhibits only slight specific differences (compare my Monograph, 1862, p. 434). Some good remarks on the structure of this typical genus and its relations to other Pylonida are to be found in Richard Hertwig's Organismus, &c., 1879, p. 52, but the true trizonal structure of the medullary shell in this genus was not recognised by him, so that his description agrees more with Dizonium. We confine here the genus Tetrapyle to those Pylonida for which Tetrapyle octacantha of J. Müller remains the determining type; the cortical shell is composed only of two perfect lattice-girdles (the transverse and lateral), between which four wide gates remain open. This structure is similar to that of Dizonium; but whilst here the medullary shell is a simple central chamber, in Tetrapyle it is a complete trizonal or Larnacilla-shell.

Definition.—Cortical shell smooth or thorny, but without large, symmetrically disposed spines.