CHAP. V. GENERA OF THE CERITHIN^. 157 



the basal canal is scarcely developed^ so that it merely 

 forms a sinus ; the epidermis is strong, and of a rich 

 fulvous brown, beneath which the shell is white. The 

 typical genus Cerithium appears to follow next, chiefly 

 disting-uished by its short and nearly straight canal : 

 the operculum is oblong and ear-shaped, and the pillar 

 smooth. These almost insensibly glide into Rhino- 

 clavis — well distinguished, however, by the shell having 

 a distinct plait on the pillar ; the basal canal fully de- 

 veloped, and turned backwards ; and by the operculum 

 being perfectly circular. These are very elegantly 

 marked shells : the colour is generally white ; and the 

 surface is often reticulated into little asperities, some- 

 thing like those on a file. To these succeed Terehralia, 

 generally of a black colour (as representing the Mela- 

 niancp), of a large size, and having the outer lip so 

 much developed, that in almost all, when fully grown, 

 it actually joins the other, and leaves only a circular 

 opening for the respiratory siphon of the animal : this 

 siphon, as shown in the admirable figures of M. Quoy, 

 assumes the shape of a radiated star, which occupies 

 the perforation, but does not protrude beyond it : the 

 tentacula are very long ; and here, for the first time, 

 we begin to see the commencement of that high deve- 

 lopement of the eyes, for which the next groups, or the 

 PterocercB and the Stromhi, are so much distinguished : 

 the operculum, as in Rhinoclavis, is round ; but there is 

 no plait upon the pillar, and the basal canal hardly pro- 

 jects. The genus Pirena * completes the circle, by 

 showing us a truncated based Cerithium, with the sinus 

 of a Pleurotoma. These shells, like Potomis, are flu- 

 viatile ; and thus we return to the point from whence 

 we began : the variations, however, are so few_, that the 

 analogies are not striking. 



(147-) That the Cerithince pass into the StromhincE , 

 by means o{ Aporrhais, we have, individually, no doubt : 

 we ground this belief more upon the similarity 



* Ferussac has most unaccountably joined this very beautiful and dis- 

 tinct type with Melanopsis. 



