Page:Scientific results HMS Challenger vol 18 part 2.djvu/607

Rh 5. Artostrobus articulatus, n. sp. (Pl. 79, fig. 16).

Shell subcylindrical, with elegant longitudinal ribs and five sharp strictures. All six joints nearly equal in length, each with three or four transverse rows of small circular pores. The middle joints twice as broad as long. Cephalis hemispherical with a conical, oblique horn.

Dimensions.—Length of the shell (with six joints) 0.16, length of a single joint 0.02 to 0.03, breadth 0.04 to 0.05.

Habitat.—Central Pacific, Stations 270 to 274, depth 2350 to 2925 fathoms.

Definition.— (vel Stichocyrtida eradiata aperta) with cylindrical shell, the upper pole of which is rounded, the lower truncate. Cephalis without horn.

The genus Lithomitra differs from the preceding genus Artostrobus in the absence of a cephalic horn, and therefore bears to it the same relation as Dictyomitra does to Lithostrobus. In many species the joints are very short, and bear only a single transverse row of pores, and since the constrictions between the joints are often very slight, Lithomitra becomes very similar to the Dicyrtide Dictyocephalus.

Definition.—A single transverse row of small circular pores on each joint. (Sometimes on the uppermost joints two or three rows.)

1. Lithomitra pachyderma, Bütschli.

Lithomitra pachyderma, Bütschli, 1882, Zeitschr. f. wiss. Zool., Vol. xxxvi. p. 529.

Eucyrtidium pachyderma, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 72, Taf. xi. fig. 21.

Eucyrtidium imbricatum, Ehrenberg, 1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 72, Taf. xi. fig. 22.

Shell thick-walled, subcylindrical, with longitudinal ribs, slightly dilated in the middle, with ten to twelve subequal joints. On the lower edge of each joint a single row of small circular pores. No external strictures. This and the following species (though separated by Ehrenberg as four different species) may be united.

Dimensions.—Length of the shell (with ten joints) 0.11; length of each joint 0.01 to 0.012, breadth 0.04.

Habitat.—Fossil in Barbados.