MUSEUM OF COMPARATIVE ZOOLOGY. 251 



Blender cylindrical and less conical shell with shorter whorls tlian S. terehralis, 

 and so far as tlu' siu'cinieiis indicate never attains the size which adult tere- 

 hralis commonly exhii)its. Otherwise the sculpture is very similar. Tiie 

 recent Sicilian shell is extremely rare and much more like tei-chraiis, which 

 Philijipi, who had received the American shell from Pfeiffer, identihed with it. 

 The recent and fossil forms seem to differ from each other in a sufliciently 

 marked way to render it most desirable to limit Ir'dineatum to the fossil to 

 which it was originally given, and retain Adams's name for the recent shell. 



The characters of the animal of Sella terebralis are such as definitely to sepa^ 

 rate it from Cerithiopsis. 



The tentacles are short and stout; the eyes are situated on their outer bases. 

 The foot is very short, indented in the median line in front, bluntly rounded 

 behind, and hardly more than one half longer than its own breadth. There is 

 a narrow mentum and the animal possesses a retractile proboscis. The oper- 

 culum is very peculiar, being subtriangular, its front edge arched, the lateral 

 margins thin, the posterior angle free, stout, hard, not spiral, but its tip curved 

 to the left. It is a little longer than wide, and when retracted the little hook 

 at the apex is caught under the twisted pillar. The opercular lobe is simple. 

 There are stout jaws composed of numerous scale-like pieces. The rhachidian 

 tooth is sub-rectangular, tridentate, the median denticle of the cusp much 

 smaller and shorter than the others; there is a squarish median boss on the 

 base below the cusp. The major lateral is broad and rather high, with a wide 

 and very short cusp, bidentate near its inner extreme. The two minor laterals 

 are narrow, long, wider at the base and bidentate at the narrow tip. 



In a general way the radula is more like that of Cerithiella metula Loven, as 

 figured by Sars (Moll. Arct. Norv., pi. vii. figs. 4 a, 41)), than any other species 

 of the Cerithiacea whose dentition I have seen figured. The details above 

 given show it to be quite different even from that form. 



Genus CERITHIOPSIS Forbes & Hanlet. 



The type of this group is C. tuhercularis Montagu, with which our most 

 common species, G. Greenii Adams, agrees in the superficial characters of its 

 soft parts. I have, however, not been able to distinguish in our species any 

 such median pore and subsequent groove as is figured by Jeffreys for G. tuher- 

 cularis (Brit. Conch., IV., pi. iv. fig. 5). The foot of the American species 

 seems longer and narrower, as do the tentacles. 



Gerilhiopsis Greenii C. B. Adams has been referred to C. tuhercularis Mon- 

 tagu, and to G. minima Brusina. It has a nucleus which, allowing for all 

 variation, is larger and less styliform than that of C. tuhercularis, while the 

 aperture of G. minima is contracted in a way different from that of any speci- 

 mens of G. Greenii I have ever examined. It would seem preferable to keep 

 them distinct, though the sculpture and its range of variation in tuhercularis 

 and Greenii are very similar. The name of G. Greenii is much older than that 



