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Show 1867.] DR. J. E. GRAY ON THE CALYPTR.-EID^E. 733 edge of the aperture, like most shells of the kind with a very large open mouth-this muscle, of a very large size and extent, leaving an impression of a horseshoe-shape, as is also the case with Patella and other shells of the short conical form. In the Calyptrceidce, on the contrary, the adductor muscle, which attaches the animal to the shell, is affixed to the inner surface of the outer lip of the shell, near the margin, as in other spiral shells -that is to say, those that are formed of an elongated shelly cone, spirally twisted round an imaginary axis. This character is constant, though the shells in the different genera assume such different forms and are sometimes so flat as to be spiral only in name ; and it is the adductor muscle that is attached in the same situation, though the outer lip may be in the form of a shelly plate or of a large cup attached to one of the sides of the simple conical cavity of the conical shell, which has only a very minute spiral nucleus, showing that the animal, which when adult forms a conical Patella-like shell, had a spiral shell on its back with a moderate-sized mouth when it was first hatched. The structure of the shells is not usually understood. The front of the last whorl is rudimentary in the entire family. It is most developed and most nearly in the usual form in the genus Trochita, and is in a much more rudimentary condition in Crucibulum and Calyptra. In the genus Mitrella the pillar-lip is reflected and soldered on itself, as in most shells with an imperforated axis; but the cavity beneath the reflexed portion in the different species and groups is gradually enlarged, until the shell has what in other shells is called an umbilicated axis. In Dispotcea and Crucibulum the cup is a very largely dilated umbilicus, surrounded by a very rudimentary front of the whorl. Anomalous as the form of the Cup-and-Saucer Limpet appears, the study of the series shows that it is only an easily understood modification of the usual form of shells. The structure of Ergcea is most peculiar. I cannot call to mind any shell showing the same peculiarities. The front of the whorls is as rudimentary as in Dispotea and Crypta; as in the genus Galerus there is a small compressed perforation extending up to the apex of the cavity; but this perforation, instead of being on the edge of a twisted central column as in that genus, is on the surface of a transverse plate. This plate somewhat resembles the front of the whorl of the genus Crypta; but in that genus the aperture and the front of the whorl are as wide as the shell, and the axis of the shell is marginal, and not marked externally. Comparing the structure of Ergcea with that of Galerus, I believe that it chiefly differs from the latter genus in that the front of the inner lip, after being reflexed over the imaginary axis of the shell as in Galerus, forming the perforation, is again reflexed and continued on to the other side of the cavity of the shell, where it is attached; the right side of the plate is broader and rounder, and the left side shorter and narrower. The generality of conchologists have so little estimation of this kind of comparison, that in studying the explanation of the structure of the genus I am laying myself open to the |
