1867. J DR. J. E. GRAY ON THE CALYPTR/EID^, 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 Fatella 

 and other shells of the short conical form. 



In the CalyptrcBidcE, 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 slielly 

 plate or of a large cup attached to one of the sides of the simple coni- 

 cal cavity of the conical shell, which has only a very minute spiral 

 nucleus, showing that the animal, which when adult forms a conical 

 P«^e//a-Iike 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 lauiily. It is most 

 developed and most nearly in the usual form in the genus Trochita, 

 and is in a much more rudimentary condition in Cruciljulum and 

 Culijptra. In the genus Mitrella the pillar-lip is reflected and sol- 

 dered 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 Cntcibulum 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 Erycea 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 colunm 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 aj)erture 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 kiud of comparison, that in studying the explana- 

 tion of the structure of the genus I am laying myself open to the 



