FAMILY 6. TURBINACEA. 163 
and the animalcule. We class her operations, as naturalists, in systematic 
or comparative order, and to enable our memory to recur with facility to 
any one of them, institute a scale of divisions, giving to every one a name ; 
but we cannot determine their exact limits,—their legitimate endings or 
beginnings ; for Nature is ever variable, never twice alike ;—she is not 
bound by mathematical exactness. Why then should the long-established 
distinction between the Trochi and the Turbines be abandoned merely 
because we have met with the intermediate forms which exhibit that gra- 
dual passage from the one genus to the other, which will sooner or later 
be found to exist between every division throughout the system? So far 
as concerns the organization or habits of these mollusks, they are scarcely 
found to differ from each other ; Linnzeus distinguished them as separate 
genera on account of the marked difference in the structure of their shells ; 
referring such as are characterized by having a conical, pyramidal shell, 
flat at the base, to the genus Trochus, and those in which it is of a tur- 
binated, top-shaped form to the genus Turbo. Both genera were followed 
in this form by Lamarck, after setting apart so many species from them 
as seemed to offer special claims for the formation of new ones ; but when 
their intermediate varieties were discovered, De Férussac as well as Des- 
hayes proposed to unite the two genera into one, whilst De Blainville at 
the same time retained them in separate families; the Trochi with his 
Goniostomata, the Turbines with his family Cricostomata. The most novel, 
but, we fear, fallacious attempt to improve the subdivision of these mol- 
lusks, is that introduced by Sowerby. After throwing the Trochi and 
Turbines together, he proposes to divide them according to the difference 
in the composition of their opercula, referring those that have a horny 
operculum to Trochus, and those that have a calcareous one to Turbo. 
Our author could not, however, have selected a character of a more un- 
certain nature, nor one which could involve the arrangement in greater 
confusion ; it is undoubtedly important to observe whether a mollusk is 
operculated or not, but we find no laws, or set of characters accompany- 
ing this strange variation in the composition of the operculum ; the Tro- 
chus virgineus, for example, which has a true pyramidal shell, is furnished 
with a very thin, horny operculum, whilst the Trochus celatus, which 
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