2l6 SHELLS AND SHELL-FISH. PARTI. 



selected as the most typical. By simply following the 

 line of affinity, we thus return to our first genus, Se- 

 nectus; for it is easy to perceive that Cidaris pethiolatus 

 has the spire and general aspect of Senectus, with the 

 truncated base and smooth surface of Cidaris. The 

 circle, in short, is complete. 



(202.) The Trochin^ naturally follows the last 

 division. The body-whorl, which in the snake-shells 

 is ventricose, is here depressed, and often flattened 

 beneath ; and this modifies the aperture, which thus 

 becomes broader than it is high, or transversely 

 oval. It is by these latter characters, also, that the 

 TrochidcB are separated from the TurhidcB, where the 

 aperture is invariably either round or longitudinally 

 oval. In the last genus of the Senectin^e, nature 

 has begun to indicate the change from a round to a 

 transverse aperture. The operculum of all the more 

 typical forms of the TrochidcB is horny; but this change 

 is effected gradually. The first genus, consequently, 

 of the TrocMncK combines the characters of both sub- 

 families. Canthorhis, in fact, has the depressed aper- 

 ture of the TrochincB, with the shelly operculum of the 

 Senectince. Canthorhis is a remarkably diversified group : 

 it contains the largest of the Trocki, properly so called, 

 as distinguished from Senectus ; but in all the shell is 

 Highly perlaceous, the aperture transversely oval, and 

 the operculum shelly.* The five types of form, or sub- 

 genera, are all recent ; and as their characters will 

 subsequently be given, we shall only, in this place, il- 

 lustrate them by general observations. All the large, 

 spinous, nodulous, and long-spired Trochi belong to 

 this genus, in which there are very few having a smooth 

 surface. It is connected to Cidaris by Rugosus and 

 Cookii; to Onustus by the sun-shells ; and to the typical 

 Trochi by our sub-genus Carinidea, into which, as we 

 suspect, will enter the Trochus Niloticus of Linnseus, — 



* If, as Sowerby mentions, the operculum of the Trochus Niloticus is 

 horny, instead of shelly, it will be the osculant species connecting Can,' 

 thorbis to our genus Trochus. 



