a 
112 BE Moutins on the genera Unio and Anodonta. 
The truth, in this matter, seems to me then, intermediate to 
two extreme propositiofis, between which M. de Blainville seemed 
to pause some years ago. Yes, I can confidently say, that there 
really exist several) species, nay, many Species in the great genus 
formed by the union of the Anodonte with the Uniones. But I 
may add with equal confidence, that authors have too greatly 
multiplied these species. Does this criticism apply also to our 
howorable correspondent, Mr. Isaac Lea? in my opinion it does. 
The more of peculiar importance that I attach to these types, s0 
well defined, so perspicuous, so distinguished by their forms, of 
which, the American authors above all, and Mr. Lea more than 
any of them, have discovered to us the existence, the more I am 
convinced of the necessity of degrading to the rank of varieties, 
and often to that of deviations, (as our learned colleague, M. Cas- 
imir Picard of é beville says, ) those variations of form typically 
finition of species, but a definition assented to by all. Defiui- 
tions, indeed, are not wanting; but common assent sanctioning 
