CHAP. VII. MELANIA. THE SUB-GENERA. 201 



semble the Planaxis mollis in size^ shape, and colour *, that 

 none but a keen-eyed naturalist would know at first which 

 was which. Both are of the same size and shape — both 

 are white — and both are covered with a pale fawn- 

 coloured epidermis ; the only difference between them 

 being the presence or absence of the minute notch at the 

 base of the pillar. We thus find the sub-genera of 

 Melania not merely to form a circle, but to represent, 

 in no unintelligible manner, the chief genera of the 

 sub-family. 



Analogies of the Sub-genera 0/ Melania. 



Sub-geiiera of < , • , /^. . Genera of the 



Melania. Analogical Characters. Melanianj^. 



Melacantha. Spire remarkably short. Paludomus, 



Melania. Spire persistent, acute. Typical. Melania. 



Potadoma. [ Spire obtuse ; shell often car'-nated 7 Melanopsis. 



i at the suture. Sub-typical. J 



r Body-whorl veniricose ; spireo 

 Hemisinus. < long ; base with a wide but not >- Cerithidea. 



C a deep notch. j 



tr t 77 ("Inner lip thickened above; spire > t>, .»,.„„ 



Melanella. J short or moderate, pointed. j PLA>fAXis. 



These remarkable analogies result from the breaking up 

 of the old genus Jfe/aw my and this is the best apology, 

 if any were needed, that we can make for so many new 

 divisions. 



(186.) The next genus, Melanopsis, is no less diver- 

 sified in its minor types, so that we may detect all those 

 which, under a different modification, exist in the last 

 genus. AVe enter this group by Melafusus — a name by 

 which we designate a remarkable fluviatile shell, having 

 the shape of a Fusus blended with that of a Melania ; 

 it differs from Hemisinus in having the base more pro- 

 duced, while the spire is shorter. Following this ap- 

 pears the typical sub-genus J/e?a«i«, — at once recognised 

 by its acutely pointed spire being longer than the aper- 

 ture, the thick enamel at the top of the inner lip, and 

 the inward curve of the pillar : the manner in which 

 the whorls are disposed on this and the next sub- 



* Sowerby's Genera, art. Planaxis, fig. £. 



