Page:Scientific results HMS Challenger vol 18 part 2.djvu/287

Rh

Definition.— (vel Monocyrtida multiradiata aperta) with flatly conical, tent-shaped or nearly discoidal shell, and numerous, simple, radial beams in its wall.

The genus Litharachnium, and the closely allied genus Cladarachnium, differ from the preceding similar genera mainly in the flat form of the depressed shell, which is not slenderly pyramidal, but more tent-shaped or nearly discoidal. The radial ribs are generally not straight, but more or less curved. The top of the tent exhibits three or four important meshes, the original pores of the cortinar septum. Litharachnium may be derived directly from Sethophormis by reduction and loss of the cephalis.

Definition.—Apex with three cortinar pores (one odd sternal and two paired cardinal pores).

1. Litharachnium tentorium, Haeckel.

Litharachnium tentorium, Haeckel, 1862, Monogr. d. Radiol., p. 281, Taf. iv. figs. 7-10.

Shell flatly conical or tent-shaped, with concave lateral outlines. Apex with three equal cortinar pores. Twenty radial ribs at nearly equal distances. Between them very numerous and thin secondary ribs, crossed by more than one hundred concentric rings. Pores square, as in all the following species. Margin truncate, circular.

Dimensions.—Shell 0.35 long, 0.8 broad.

Habitat.—Mediterranean (Messina).

2. Litharachnium araneosum (Pl. 55, figs. 8, 10).

Shell flatly conical, nearly discoidal, with straight lateral outlines. Apex with three unequal cortinar pores (one odd smaller sternal and two paired larger cardinal pores). Twenty radial ribs at unequal distances, with numerous intercalated secondary ribs. Margin ciliate.

Dimensions.—Shell 0.18 long, 0.32 broad.

Habitat.—Central Pacific, Station 271, surface.