OCR Text |
Show 998 DR. J. E. GRAY O N CATILLUS. [Dec. 12, notch on its left margin (Figs. 3 a, 3 b). Shell elongate, mouth elongate, narrow. Fig. 3 a. Fig. 3 b. Fig. 3 a. Outer surface. Fig. 3 b. Inner surface. The operculum has a very thin brittle shelly plate, which is separated from the horny (or true) operculum by a distinct straight line running from the centre of the right side of the operculum to the outer side rather above the base. The outer surface of the shelly plate is slightly convex, smooth, very finely concentrically striated, with a diverging rib from the nucleus to the upper edge, and with another more distinct linear rib on the right margin, which ends in a slender spine; the space between the diverging ribs is depressed and striated, like the rest of the surface. According to the figure of the animal in Adams's ' Genera' (p. 386, t. 42. f. 4), the end of the operculum is produced beyond the end of the foot. STENOPOMA, n. g. STENOPOMA LINEATUM. (Operculum, Fig. 3.) Navicella lineata, Lamk. E. M . t. 456. f. 2; Reeve, t. 8. f. 31. N. tessellata, Lamk.; Reeve, t. 6. f. 27. Septaria navicula, Ferussac, Bull. Hab. Philippine Islands. Lamarck divided these shells into two species, according to the coloration, calling one lineata and the other tessellata ; but the two systems of coloration gradually pass into each other in the different specimens; and some specimens present the two kinds of coloration each well marked: for example, one was C. lineata when young, and became C. tessellata afterwards; so that the upper part of the shell is of one species, and the margin of tbe other. The specimens from the same locality present considerable variation in the general form of the shell, some being much narrower and more convex than others, and also in the form, position, and extent of the hinder lip. These shells have a general external resemblance to the compressed Limpets (Patellee), that live in the cavities which they eat out at the base of the roots of Algce, or on the cylindrical stems of those plants. They do not appear to derive their form, like those shells, from adapting themselves to the form of the body to which they happen to be attached; and, indeed, Mr. Cuming states that the specimen which he procured in the Philippines lived on stones, like the Catilli. Though they are compressed and oblong, the base of the cavity is flat and even, as much so as the circumference of |