12ti SHELLS AND SHELL-FISH. FART I. 



divided into three other circles, — the first of which, 

 Voluta, is typical ; the second, Cymhiola, is sub-typical ; 

 and the third, which contains Harpula, Volutilithes , and 

 Scaphella, constitutes the aberrant circle. This, in fact, 

 is only in conformity with all other natural groups in 

 the Vertehrata ; so that, if the principles upon which 

 we have now arranged the Volutince are radically de- 

 fective, so also are those in the innumerable groups 

 that have been thus tested and verified in ornithology, 

 ichthyology, &c. : for it is logically certain, that unity of 

 plan cannot exist in the animal world, if one and the 

 same principle of variation is not fundamental, and does 

 not pervade all its parts. 



(116.) The MiTRiNiE, or mitres, no less than the 

 volutes, are characterised, in the great majority of the 

 species, by very distinct and tangible characters ; but 

 so soon as we reach the extreme limits of each, and 

 look for those marks by which we have been accustomed 

 to separate, with ease, the two groups, we find some 

 of them gradually disappear, and others so modified 

 and interchanged as to render it very difficult for an 

 ordinary naturalist to draw any positive line of demarc- 

 ation, — at least, on those principles of absolute division 

 insisted upon by some writers. It has, for instance, 

 been thought that a shell having the spire papillary, 

 and the lower plaits upon the pillar largest, was un- 

 questionably a volute: and yet we have, in Volutilithes, 

 a whole assemblage of shells whose apex is acute ; and 

 in another group, Scaphella, the lower plait is always 

 the smallest. Among the mitres, on the other hand, 

 we shall find more than one with a papillary spire, and 

 others where the lower plaits are as large as the upper. 

 Hence it folloAvs, that if no groups are to receive ge- 

 neric or family names, but such as are distinctly sepa- 

 rated, and are destitute of these intervening links, the 

 old genera of Voluta and Mitra must again be united. 

 Nor should we stop here : Mitrella so connects the 

 mitres with the olives, that those, too, must be ab- 

 sorbed in this gathering conchological snowball ; and 



