Habitat.—Central Pacific, Station 271, depth 2425 fathoms.

3. Conchonia tetrodon, n. sp.

Conchura tetrodon, Haeckel, 1882, Manuscript.

Shell subspherical, with two nearly equal hemispherical valves, which are very thin-walled and similar in structure to those of the Cœlodendrida, with very irregular roundish pores of different shapes and sizes (compare Pl. [121], fig. 3). Lateral margins of the valves with very numerous and irregular, thin, bristle-shaped teeth, similar to those of some Cœlographida (compare Pl. [127], fig. 8). Aboral hinge with two equal, conical, caudal horns, which are straight, parallel, and half as long as the shell. Two similar straight conical horns are opposed on the poles of the sagittal axis, and arise from the apex of the two valves. This remarkable species may perhaps better represent a separate genus, Conchura, forming a direct transition to the ancestral form of the Cœlodendrida, Cœlodoras; it differs from the latter in the absence of a galea or hollow conical cupola on the apex of each valve, and in the solid, not hollow structure of the horns.

Dimensions.—Diameter of the shell 0.24, length of the two sagittal horns 0.1, of the two caudal horns 0.12.

Habitat.—Indian Ocean, Cocos Islands (Rabbe), surface.

Genus 725. Conchopsis,[[341]] Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6.

Definition.—Concharida with dentate lateral margins and a sharp sagittal keel of the compressed valves, without horns on the hinge.

The genus Conchopsis and the following Conchoceras differ from the other Concharida in the strong lateral compression of the shell, so that each valve is provided in the sagittal plane with a sharp prominent keel, comparable to the dorsal and the anal fin of fishes. These compressed shells are in general twice to three times as large as the more roundish and keelless shells of the five preceding genera. The sculpture of the fenestrated valves is extremely elegant. Conchopsis possesses at the aboral hinge not the two prominent caudal horns, which mark the following genus Conchoceras, but in some species a peculiar ligament connects the aboral ends of both valves.

1. Conchopsis orbicularis, n. sp. (Pl. [125], fig. 3).