146 SUPPLEMENT 
the methodical distribution of these envelopes, in which, after 
having proved that the knowledge of them may suffice, he 
nevertheless remarks that that of the animals is indispensable 
to form a complete system of conchology, and a natural distri- 
bution of shells. We do not, however, find that he carried this 
principle into execution; at least there is no mention of his 
having done so, in the extract given from his memoir by the 
secretary of the Academy. 
In 1766, Guettard, a member of the same society, was the 
first who put in practice what Daubenton merely hinted; for 
in a very detailed article, inserted in the Acts of the Academy, 
the covert object of which appears to have been a criticism of 
some observations of Buffon, at the commencement of his 
description of the ass, on species and its distinction,—not only 
does he establish on indubitable principles the necessity, in 
the classification of shells, of having recourse to the animal 
which they contain, and of which they form a part, but he 
characterizes a certain number of genera, at least among the 
univalves, extremely well. 
Although in this memoir Guettard tells us that the genera 
of bivalves ought also to be susceptible of being equally cha- 
racterized from the animal, he confesses that his observations 
on them have been too few to enable him to make the trial; 
but he very well discriminates how far the division of shelled 
mollusca into terrestrial, fresh-water, and marine, is exact. 
He likewise pays great attention to the presence or absence of 
the operculum. 
These new observations of Guettard doubtless determined 
d’Argenville, in the second edition of his Conchology, in 1757, 
to add a great number of figures, unfortunately very bad, of 
animals, under the name of zoomorphoses, but without serving 
any purpose in the characters of his genera of shells, which 
had been justly criticised by Guettard, in the memoir just 
noticed. 
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