Page:Scientific results HMS Challenger vol 18 part 2.djvu/847

Rh 7. Conchidium magasella, n. sp.

Shell laterally compressed, with two equal valves, very similar to the preceding species, but differing in the following characters:—Proportion of the three axes = 6 : 4 : 3. Margins of the valves with smaller and more numerous teeth; on each side of one valve sixteen to eighteen short conical teeth. Pores smaller and more numerous, about as broad as the bars.

Dimensions.—Length of the shell 0.24, height 0.16, breadth 0.12.

Habitat.—Indian Ocean, Madagascar (Rabbe), surface.

8. Conchidium productum, n. sp.

Shell laterally compressed, prolonged, with two equal valves, similar to the two preceding species, differing in the following characters:—Proportion of the three axes = 3 : 2 : 1. Margins of the valves smooth in the anterior and posterior quarter, dentate in the middle lateral half; on each side of one valve ten to twelve strong conical teeth. Horns of the aboral hinge prolonged, conical, half as long as the shell, the ventral somewhat larger than the dorsal. Pores regular, circular, twice as broad as the bars.

Dimensions.—Length of the shell 0.25, height 0.15, breadth 0.08.

Habitat.—North Atlantic, Station 354, surface.

Definition.— 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