200 SHELLS AND SHELL-FISH. PART I. 



Melania amarula and setosa are typical examples. The 

 spire is short, and the volutions coronated ; the inner lip 

 is merely a thin glazing ; and the base of the piUar is flat- 

 tened. It will be remembered that Hemimitra is the 

 coronated type of Paludomus, and is so like Melacan- 

 tha on a superficial glance, that, but for the difference in 

 their apertures, they might be taken for species of the 

 same genus. From these we are gradually led to the 

 typical division of Melania, whose spire is often as long 

 as in Turritella. These shells are much lighter than 

 those of the next group, Potadoma, which are more solid 

 in their substance; the base of the pillar is not depressed 

 or broad, but convex and comparatively thin ; it is like- 

 wise, in most of the species, much straighter, while the 

 base of the outer lip is more dilated. All the species we 

 have yet seen of this sub-genus are decollated. We 

 regard, as the most typical character, a slight thickening 

 of the inner lip, particularly at its upper part, because 

 this is never seen in the last group. Our next sub-genus, 

 Hemisinus, has hitherto been placed with Melanopsis, as 

 the base of the aperture is notched : the type is the Me- 

 lania lineata *; in its general form, indeed, it resembles 

 the preceding shells ; but the body-whorl, although more 

 ventricose, is yet contracted at its base, the outer lip but 

 slightly dilated; and the inner lip (now for the first time 

 clearly developed) is complete, — that is, it extends from 

 one extremity of the aperture to the other. The lip of 

 the columella, or pillar, instead of turning inward, turns 

 outwards ; that part which is covered by the inner lip 

 being straight ; while the sinus, or notch, at the base, 

 is even wider than in Melanopsis, to which this type 

 obviously leads. But before we cross the threshold thus 

 opened to us, we must notice Melanella, — another type 

 resembling Hemisinus in shape, but having the aperture 

 perfectly entire ; the inner lip is much developed, and its 

 upper angle has a callosity precisely like that of Planaxis. 

 Sq remarkably, indeed, does one of these MelanellcE re- 



* Gray, in Griff. Cuv. pi. 13. fig. 4. 



