Page:Scientific results HMS Challenger vol 18 part 2.djvu/259

Rh

Definition.— with the basal mouth of the shell open (vel Monocyrtida triradiata aperta).

Definition.— (vel Monocyrtida triradiata aperta) with three lateral ribs prolonged into three terminal feet. Apex with a horn.

The genus Tripocalpis opens the series of Archipilida, or of those Monocyrtida in which the monothalamous shell exhibits a distinct triradial structure, and a simple open mouth, without cortinar lattice. Tripocalpis is probably the most primordial of the Archipilida, and has three lateral ribs, which are prolonged into three free terminal