CHAP. VII. TROCniD^. — CANTHORBTS. 225 



clue to the meaning of this variation, it might appear a 

 questionable group. The only one of these analogies 

 on which we have any doubts, is that between Carini- 

 dea and Trochiis : we are, in fact, at a loss to know 

 whether Trochus Niloticus and Turbo Pica are the real 

 types of Carinidea, in which case they would open a 

 passage from the large Canthorhi to the small and 

 moderate-sized shells of our genus Trochus: the strong 

 and unquestionable affinity, however, of Carinidea con- 

 cavus* to our sub-genus Chlorostonius, although the 

 one is a large and the other a small shell, cannot" be 

 disturbed. But, on the other hand, even if the passage 

 from Canthorhis to Trochus is made by Niloticus and 

 Pica, the difference is one of very inferior moment. 

 Two things are certain : one, that Canthorbis is the 

 intermediate group, which connects the TrochincB to 

 the Senectina; the other, that it equally connects our 

 genera Trochus axvA Onustus: for every conchologist will 

 perceive that Tubicanthus runs into Cidaris by means 

 of C. rugosus and Cookii. There is, however, another 

 analogy belonging to Canthorbis. too remarkable to be 

 passed over : it is, in its own group, what Cerithium 

 is in the circle of the Strombidce: this is shown in its 

 twisted and outwardly-curved pillar, as well as the 

 numerous volutions, and the consequent length of the 

 spire. According to this view, Pyramidea and Lam- 

 prostoma would be the types, — since they are the most 

 conical and elevated of all the others. 



(210.) We now come to the sub-genera of the two 

 typical groups, Trochus and Monodonta. It has been 

 seen that each forms a circular group ; and the pre- 

 ceding observations will, in some measure, have pre- 

 pared the reader for the following general exposition 

 of their sub-genera. The genus Margarita of Leach 

 appears to us a purely artificial group, partly ccmposed 

 of our umbilicated Trochi, and of the true Calliostonice ; 

 we have therefore not adopted it. 



* Our specimen, fortunately, possesses its operculum, and has never been 

 cleaned. 



