CHAP. VII. TROCHIN^. THE GENERA. 21? 



a very remarkable shell : out of hundreds, we have 

 never yet seen a specimen, however large, which had a 

 perfectly formed mouth, so that we feel somewhat un- 

 decided as to its precise station. In a young state, the 

 basal volution is often perfectly flat, and even concave ; 

 and this gives the margin precisely the same carinated 

 edge as belongs to Carinidea ; but when more advanced 

 it becomes convex, as in the genuine Trochi, thus 

 uniting, at different periods of its growth, the characters 

 of the two groups between which it appears to stand. 



(203.) Having now shown, by the foregoing details, 

 the union of the Senectinte and the Trochince, we 

 shall briefly characterise the remaining genera of the 

 sub-family we are now upon, and then notice their sub- 

 genera. Canthorbis has already been defined : following 

 this is Trochus, properly so called, where the umbil- 

 icus, if it exists, is never toothed, and very rarely 

 channeled. In Monodonta, these characters are reversed; 

 the umbilicus, if present, is always either toothed or 

 channeled, and the aperture striated. The fourth genus 

 is Solarium, where the umbilicus is so large as to reach 

 to the apex, the pillar is absent, the shell nearly dis- 

 coid, and the aperture without any defined lip. The 

 last genus is Onustus, long ago separated by Humphrey, 

 to include those singular and half-formed shells, called 

 by collectors,, Carriers. Of the animals by which these 

 are formed, we as yet know nothing ; but their shells are 

 composed partly of the usual calcareous substance, and 

 partly of little stones or fragments of other shells, 

 which the animal gathers up and incorporates on the 

 outer surface -of its own habitation. A gradual series 

 of intervening forms unites this group on one hand to 

 Solarium, and on the other to Canthorbis ; so that the 

 whole of the Trochince, being united into one circle, 

 constitute a natural and perfect group. We shall now 

 notice each of these genera in detail. 



(204.) Having already spoken of CANTHORBIS, we 

 pass to the second genus, TROCHUS. Were we to 

 make this an artificial group, its definition would be very 



