CHAP. V. GENERA OF THE PLEUBOTOMINuE. 155 



to which we have retained Lamarck's original name of 

 Clavatula ; it has the long narrow slit of Pleurotoma, but 

 with a very short canal. Following this comes Clavi- 

 cantha, having the canal equally short ; but the sinus, or 

 notch, instead of being linear and long, is short and 

 wide, the surface is rough, and the whorls either coro- 

 nated with prickles or with compressed nodules resem- 

 bling spines. The last genus is Tomella, which has the 

 spire and canal nearly equally fusiform ; but the former 

 is of very few whorls, and the inner lip is considerably 

 thickened within, where it joins the outer lip. In these 

 five genera, all the varied species of this group will 

 arrange themselves : the two typical are recognised by 

 the length and narrowness of the sinus, which, in the 

 three aberrant genera, is short and wide. Clavicantha 

 is a very remarkable form, inasmuch as it seems pro- 

 bable that echinata and auriculifera * are fluviatile shells. 

 Lamarck describes them as PleurotomcB; and not having 

 seen them, we take his authority for the fact ; but we 

 might almost question whether they do not belong to 

 Potomis, which is the first genus in the Cerithince, 

 so intimately do these sub-families appear connected. 

 Nearly the whole of these shells, like the mitres, are 

 scarce ; and yet the species are numerous. They chiefly 

 come from the Indian seas, and other warm latitudes. 



(145.) The situation and the genera of the Pleuro. 

 tomince, thus determined by analysis, offer some very 

 curious analogies which merit our special notice. The 

 sinus, which gives them the appearance of having the 

 outer lip dilated, although it is really not so, is only a 

 modification, as we believe, of the wider sinus nearly 

 in the same part of the outer lip in S trombus; and 

 these, again, are but different modifications of the same 

 part in the typical cowries, where the outer lip forms a 

 sinus before it joins the umbilicus or depression of the 

 spire. That this fissure is intended to admit the 'pro- 

 trusion of some particular organ of the animal, is un- 



* Ency. Meth. pi. 439. figs. 8. 10. 



