Genus 724. Conchonia,[[340]] n. gen.

Definition.—Concharida with the lateral margins of the valves dentate, without sagittal keel, but with an apical horn on the poles of the sagittal axis, and with two caudal horns on the hinge (a dorsal and a ventral).

The genus Conchonia is closely allied to the preceding Conchidium, its ancestral form, but differs from this and from all other Concharida in the development of horns on the poles of the sagittal axis. These are probably of great morphological importance, since they represent the beginnings of the hollow tubes arising from the poles of the sagittal axis in all Cœlodendrida and Cœlographida. In one of the three observed species each valve possessed an apical or sagittal horn, whilst in the two other species one valve only was provided with a horn. Since I observed one specimen only of each species, I cannot say whether this difference is important and of constant generic value.

1. Conchonia diodon, n. sp. (Pl. [124], figs. 10-12).

Shell laterally compressed, with two very unequal valves. Dorsal valve smaller, hat-shaped, on the apex with a fenestrated protuberance which is similar to the galea of the Cœlodendrida, and bears a short, conical, backwardly-directed horn. Ventral valve larger, boat-shaped, without apical horn. Aboral hinge with two pyramidal, horizontal, caudal horns of different sizes, the dorsal smaller than the ventral. Lateral margin of each valve on one side with twelve to fourteen strong conical teeth (fig. 12). Lips of the narrow mouth thickened (fig. 11).

Dimensions.—Length of the shell 0.3, height 0.27, breadth 0.21.

Habitat.—Tropical Atlantic, Station 342, depth 1445 fathoms.

2. Conchonia triodon, n. sp. (Pl. [124], figs. 13, 14).

Shell laterally compressed, with two very unequal valves. Dorsal valve (fig. 14) larger, hat-shaped, on the apex with a large pyramidal horn which is half as long as the shell, curved and directed backwards. Ventral valve (fig. 13) smaller, boat-shaped, without apical horn. Aboral hinge with two pyramidal caudal horns of different sizes, the dorsal horn twice as long as the ventral. Lateral margin of each valve on one side with twelve to fifteen conical teeth. Perhaps the larger horned valve (fig. 14) may be the ventral, and the opposite smaller hornless (seen from above in fig. 13) the dorsal valve.

Dimensions.—Length of the shell 0.21, height 0.17, breadth 0.12.