Page:Scientific results HMS Challenger vol 18 part 2.djvu/881

Rh 2. Cœlostylus flabellatus, n. sp.

Shell-mantle one and a half times as long as broad, very similar to that of the preceding species. It differs from this mainly in the peculiar form of the twenty-four terminal coronets, which are flabellate and very similar to the terminal branches of Cœlodendrum flabellatum (Pl. 121, figs. 5, 6). I formerly supposed, therefore, that both forms belonged to one and the same species. But though I had no complete specimens of the two species, I was afterwards led to the opinion that one and the same peculiar form of terminal branches is here produced by adaptation to similar conditions in two very different genera. Each coronet is a flat flabellum, placed in a meridian plane, and composed of eight pairs of spinulate branches as figured, ''loc. cit.'', in figs. 5 and 6. The cap-shaped distal end of each finger is armed with eight to twelve recurved teeth.

Dimensions.—Length of the shell 3.8, breadth 2.6.

Habitat.—North Pacific, Station 252, surface.

Definition.— with an odd frenulum on each galea and an outer lattice-mantle, armed with fourteen styles (one odd and six paired styles on each valve).

The genus Cœloplegma, distinguished by the possession of fourteen coronal styles, is closely allied to Cœlodecas, but differs from it in the development of a new pair of styles on each valve. These are placed between the odd nasal and the paired frontal styles, are directed forwards, and may therefore be called pectoral styles, corresponding to those of Cœlostylus.

1. Cœloplegma murrayanum, n. sp. (Pl. 127, figs. 1-13.).

Shell-mantle about as long as broad, its frontal perimeter subcircular, or slightly pentagonal, with five convex sides, its sagittal perimeter also nearly circular. Odd nasal style with six to eight, the paired pectoral styles with two or three, lateral styles with three to five, and tergal styles with six to nine pairs of branches. The central capsule constantly contains masses of crystals (figs. 4-7). Terminal coronets (on the free distal ends of the styles) three times furcate, each with eight simple and short fingers. This interesting species, discovered by Dr. John Murray in August 1882, during the expedition of H.M.S. "Triton," in great numbers in the Gulf Stream, off the Færöe Channel, is very variable and connected by numerous transitional forms with the following closely allied species. (Compare the following note.)

Dimensions.—Length of the shell 1.6 to 2.2, breadth 1.5 to 2.1.

Habitat.—North Atlantic, Gulf Stream, off the Færöe Channel, in depths between 40 and 200 fathoms, John Murray.