1! 



112 Des Moulins on the genera Unio and Anodonta. 



The truth, in this matter, seems to me then, intermediate to 

 two extreme propositions, 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 Anodontce 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 

 honorable correspondent, Mr. Isaac Lea ? in my opinion it does. 

 The more of peculiar importance that I attach to these types, so 

 well defined, so perspicuous, so distinguished hj 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 Abbeville says,) those variations of form typically 

 the same, those specimens from different localities, which are not 

 identical in the detail of parts, which are neither precisely this, 

 nor exactly that, but which possess a clear and undeniable rela- 

 tionship with such or such a type perspicuously defined. I think 

 that a certain number of Mr. Lea's species would be extinguished 

 by a reform based upon this principle. 



But to propose this reform in detail, we should have under our 

 eyes the collection of Mr. Lea itself, probably the richest in the 

 world ; or at least a collection very numerous in species and spe- 

 cimens, identified by comparison with all the published figures 

 of authors. To establish such reform irrevocably, still more is 

 necessary ; we must have what at this moment may be consid- 

 ered as impossible, the com-parative anatomy of all the pretended 

 species established by authors. Then only could we be absolutely' 

 sure of viewing the question in all its phases. I think I may 

 affirm that we should then find anatomical differences between, 

 all true species, and we would rank without scruple among /orws- 

 or varieties all those diversities as to length, shortness, &c. which 

 exert no influence upon the internal structure, the number, or the 

 functions of the organs and integuments. Thus, better than in 

 any other way, should we approach the solution of that important 

 question, what is species 7 a question which so much occupies,! 

 and still more divides philosophers. Every body demands a de- 

 finition of species, but a definition assented to by all. Defini- 

 tions, indeed, are not wanting ; but common assent sanctioning 



