Page:Scientific results HMS Challenger vol 18 part 1.djvu/1051

Rh 14. Lychnaspis cataplasta, n. sp.

Parmal pores very small, circular, half as broad as the bars, and much smaller than the irregular sutural pores. By-spines zigzag, as long as the diameter of the shell. Radial main-spines very thin and long, needle-shaped, cylindrical, five to six times as long as the diameter of the shell. Sutures perfectly obliterated. (This stunted species is one of the smallest of the Dorataspida.)

Dimensions.—Diameter of the shell 0.05, parmal pores 0.0015, sutural pores 0.01, bars 0.002.

Habitat.—Antarctic Ocean (off Kerguelen Island), Station 149, surface.

Definition.— with twenty plates, which are perforated by one hundred and sixty to three hundred or more parmal pores (in each plate four crossed aspinal pores, and around them four to twelve or more coronal pores). Surface without by-spines.

The genus Icosaspis and the closely allied Hylaspis differ from all other Tessaraspida in the increased number of the parmal pores. Whilst this number in all other genera is eighty (only four crossed pores in each plate), here it amounts to one hundred and sixty to three hundred or more (sometimes more than a thousand); in each shield four primary, crossed "aspinal pores" being surrounded by a circle of four to twelve or more "coronal pores." The number of sutural pores in these two genera is also increased.

Definition.—Condyles of the neighboring plates connected by permanent open sutures; therefore the whole shell composed of twenty separated pieces of acanthin.

1. Icosaspis tabulata, n. sp. (Pl. 136, fig. 2).

Parmal meshes all of nearly equal size and form, square, four times as broad as the bars, little larger than the triangular or polygonal sutural meshes. In each plate fifty to seventy (regularly sixty-four) quadrangular pores, viz., four primary square aspinal meshes, forming together a regular square surrounded by two to three coronas of rectangular (not quite regular) coronal meshes (six to eight in each transverse row). Radial spines tetrapterous, prismatic, with four thin and broad wings, from which arise the crossed bars between the four primary pores. Outer part of the spines longer than the inner. Commonly the condyles of the plates are only contiguous; sometimes they grow together, and this form approaches Icosaspis tetragonopa.

Dimensions.—Diameter of the shell 0.25 to 0.3, of the pores 0.02, bars 0.005.

Habitat.—North Pacific, Station 244, depth 2900 fathoms.