168 MOLLUSCA. 
beneath, cream-coloured, tinted bluish; head beneath, azure; mouth 
yellow. 
The shell represented is about half grown, and coloured for the 
most part black, mottled with ash-coloured blotches, as we find in the 
earlier stages of the majority of specimens. 
Figure 196, 196 a, two views of the animal, with the dark variety 
of the shell. 
TROCHID. 
No group of shells remains in a more unsatisfactory state of adjust- 
ment than that of the Trochide, including among others the genera 
Trochus, Turbo, Monodonta, Delphinula, and Littorina. Some have 
maintained that all these, and several more, should be held as well- 
characterized genera. Others, and especially M. Deshayes, after a more 
extended examination of the animals, and finding their organization 
to be essentially alike, would incorporate the first four at least, into 
one great genus, subdivided into sections. Swainson, Philippi, and 
Gray, have each proposed extensive subdivisions, all of which, how- 
ever, are founded on characters of the shell alone. Still, the relations 
of the several members of this group remain unadjusted ; and the diffi- 
culties, both as to generic and specific distinctions, seem to have been 
so numerous that no one has ventured to undertake the task. Concho- 
logists have therefore passed by this field with the hope of by and by 
returning to it better qualified, by the exploration of less doubtful 
groups, to cope with the difficulties; but other fields have proved so 
fruitful that no one has yet arrived at this. 
Deshayes indicates the trae method to be adopted, when he remarks 
that we must scrutinize as many of the animals of these shells as 
possible, and ascertain if there be not some zoological characters to 
circumseribe genera which cannot be limited by the shell alone. As 
a contribution to this end, all the figures of animals of this family, 
made during the Expedition, are here given; and the result to which 
we have arrived, from a comparison of these figures, and such others 
as we have had access to, is, that there are zoological characters im- 
portant enough, and constant enough, to justify generic subdivisions ; 
