CHAP. VII. BIELANIAN^E. THE GENERA. 199 



genus Anculosa of Say (fig. 31. a}*, and almost resemble 

 nerits. Notwithstanding the thickness of these shells, the 

 outer lip is unusually thin, and the inner is like that of a 

 Purpura, being broad and flat- 

 tened. One or two species now 

 before us are so like young ma- 

 v \ ^y\\ r * ne turbos, that none but a 

 y practised eye would distinguish 

 them. Theyare,however,strictly 

 fluviatile shells, having a horny 

 operculum ; and are abundant in the Ohio. The next sub- 

 genus is Paludomus proper (6) : they differ from Anculosa 

 in being sub-spiral like the JBulimi, and in having both 

 lips thickened, although not margined by a rim ; the 

 outer one is slightly reflected and crenated, and the in- 

 ner perfect and convex : these seem peculiar to the Indian 

 rivers. In the next sub-genus, Hemimitra, the general 

 form of Paludomus is preserved, but the whorls are 

 coronated by spines. The whole are readily distinguished 

 from the next genus, by not having the outer lip dilated 

 at its base, the inner lip complete, the aperture wide, and 

 the spire always shorter than the body-whorl.t 



(184.) The true genus MELANIA comes next. Amid 

 the great diversities of forms it contains, even as we 

 now intend to restrict it, there may be detected four, if 

 not five, types or sub-genera; yet, with one exception j, 

 they are all possessed of an entire aperture, and are more 

 or less spiral. The animal, according to Cuvier, has a 

 proboscis-like mouth, and the two tentacula bear the eyes 

 half way on their external side ; the aperture is always 

 oval, the outer lip thin, and is generally much dilated at 

 the base. The five types of form, or sub-genera, appear 

 to be these: 1. Melacantha; 2. M elania ; 3. Po- 

 tadoma ; 4. Hemisinus ; and, 5. Melanella. 



(185.) The first type which meets us after quitting 

 Paludomus is Melacantha, of which the well-known 



* I consider this, however, as a form between Paludina and Paludomus. 

 t Melanin conica, globulosa, and retusa, of Mr. Gray, Griff. Cuv. pi. 14., 

 belong to this group. 

 J The sub-genus Hemisinus. 



o 4 



