Page:Scientific results HMS Challenger vol 18 part 2.djvu/303

Rh Prodromus, comprises those in which the lattice-shell is quite simple, without transverse constriction, and without radial apophyses. The two subfamilies differ in the shape of the basal mouth, which in the Archicorida is a simple wide opening, but in the Archicapsida is closed by a lattice-plate.

Only a few species of this family were formerly known. Ehrenberg, in 1838, founded upon these the genus Cornutella, one of the three oldest genera of Polycystina (Cornutella, Lithocampe, Haliomma). In 1862 I described some living forms, with a central capsule, as Cyrtocalpis. The Challenger collection contains a large number of new genera and species.

Probably the family Cyrtocalpida is an artificial group, comprising two or more different subfamilies of very different origin. The Cornutellida (Cornutella, Cornutanna) have probably been derived from the Sethocorida by loss of the cephalis, so that their conical shell represents a thorax alone. The Mitrocalpida on the other hand (genera 528-531) may be originally simple ovate shells (like Gromia and Lecythium), arising independently from the Nassellida. The Archicapsida (Halicapsa, Archicapsa) are probably derived from the Zygospyrida (Dictyospyris, Circospyris) by loss of the sagittal ring and constriction. This is nearly certain, when the three or four typical cortinar pores appear in their basal plate. Some forms of Halicapsa may be easily confounded with some forms of Prunoidea (Lithapium).

Those Cyrtocalpida, which possess a central capsule with three or four lobes, are probably derived from Tripocyrtida by loss of the three feet, or from Sethocyrtida by loss of the cephalis, since the lobes indicate the original presence of cortinar pores and of a cephalis. Those Cyrtocalpida, however, in which a simple ovate shell encloses a simple central capsule without lobes, may be original "Monocyrtida eradiata," without relation to any radial ancestral forms.