CH. VII. SUB- GENERA OP MEL ANOPSIS. - ANALOGIES. 



203 



thickened above ; the pillar is straight, and the notch 

 at the base nearly as wide as that of an ordinary Pleu- 

 rotoma ; the whole shell is marked with regular longi- 

 tudinal plaits, and coronated on the suture by a row of 

 tubercles. Our specimen, although in bad condition, is 

 still partially covered with a brown epidermis, beneath 

 which the shell is of a livid colour : the aberrant 

 species of Canthidomus, as C. costatus and Owenii, pass 

 into Melatoma, and complete the circle. 



Analogies of the Sub-genera of MELANOPSIS. 



Analogical Characters. 



Sub-genera 



of 

 Melanopsis. 



Melafusus. 



Melanopsis. Baseof the aperture contracted. Potadoma. 

 Melanithes. Tip of the spire obtuse. Melania. 



Sub-genera Genera 

 of of the 



Melania. MELANIAN.K. 



MELANOPSIS. 

 MELANIA. 



"" 



"] 



The use of the last, or additional column, which con- 

 tains the genera of the entire sub-family, is chiefly 

 for the purpose of showing that Melatoma, while it 

 preserves its analogy to Pleurotoma, agrees also with 

 Planaxis in having the base notched, and with Mela- 

 nella by its thickened inner lip. 



(187.) The next genus is' that of Cerithidea. We 

 have now come to the cyclostiform type, which, with 

 the elongate form of Scalaria, has an effuse and circular 

 aperture, with the outer lip dilated into a broad fringe, 

 and a very short notch at the base. The lightness of 

 these shells would seem to indicate that they were flu- 

 viatile ; but as they are slightly variegated, and have 

 no epidermis, we should not be surprised at their being 

 found in the sea, more particularly as this appears to 

 be the point where the series of fluviatile Testacea 

 terminates, and the marine commences. Nevertheless, 

 the great change from Melanopsis to Cerithidea is not 



