CHAP. VII. MELANIAN^. GENERAL RE3IARKS. 205 



hidce which stand upon the very confines of the phyto- 

 phagous circle, yet still within its limits : it therefore 

 partakes much more of the tribe which Nature is about 

 to enter upon, than of that she is on the point of quitting; 

 and, consequently, not only the animal, but even the 

 shell, is so fashioned as to exhibit this preponderance to 

 the first rather than to the last group. On this broad 

 principle do w^e account for the indication of a basal 

 channel seen in all the sub-genera of Melanopsis, in Pla- 

 naxis, and in Cerithidea. Nay, to such a refined point 

 is this principle of gradual developement carried, that 

 we hardly know, at present, where to draw a line of 

 demarcation between the TurhidcB and the Stromhidcp ; 

 not, of course, in their pre-eminent types, but in those 

 which are aberrant. Cerifhiion is in one, and Ceri- 

 thidea in the other. Their typical forms are easily dis- 

 tinguished. But in which of these are we to place the 

 apparently anomalous sub-genus Cerlphasia ? and how 

 delicate and refined are the characters by which this is 

 proposed to be detached from the fiuviatile PotomidcB of 

 Brongniart ! It is here, then, rather than among any 

 other of the Melaniance, that we should say the tw^o 

 tribes actually unite. All modern writers, indeed, have 

 perceived this ; and some have gone so far as to unite 

 all the sub-genera of Melania and Melanopsis, and 

 many of the CerithincE, into one genus. It is quite 

 clear, however, that if this principle be acted upon, the 

 greater our knowledge of the Testacea is enlarged, the 

 more must the number of our genera or divisions — call 

 them what we will — be reduced. New species bring 

 new modifications of forms ; and these, filling up inter- 

 vals, and softening down differences, wnll so blend groups 

 which are noAv in some degree detached, that the whole, 

 in process of time, will present but one continued chain 

 of gentle gradations. No " well-marked divisions," in 

 the sense which the term has been used in, could by 

 any possibility exist. Our tribes and families, genera 

 and sub-genera, v/ould melt, one after the other, into 

 the general mass — they would be abolished — and our 



