114 Des Moulins on the gejicra Unio and Anodonta. 



fiuviatilis ; such as the form of the foot or of the appendices of 

 the mantle in the Unio radiatus, &c. 



Secondly, we will take for our guide that very important pub- 

 lication which Mr. Lea is at present making of observations made 

 by Dr. Kirtland and himself on the dioicity of the Naiades, and 

 on the variations of form which it produces in individuals of the 

 same species, and we should thus arrive at the certain and ra- 

 tional extinguishment of many species that authors have estab- 

 lished, as Mr. Lea admits, solely upon their external character. 



Thirdly, we will apply most rigorously the luminous and solid 

 principles that M. Casimir Picard, of Abbeville, is going to pub- 

 lish, 1 hope, in a short time, upon the deviations or pathological 

 malformations of the Naiades ; and many more species would 

 disappear in the sieve of this physiological scrutiny. 



Fourthly, we will examine the laws of the geographical distri- 

 bution of animals in the different classes, and, governed by anal- 

 ogy, we will not admit cosmopolitan species, except with the 

 greatest reserve ; but we will take equal care not to give too much 

 specific importance to a difference of localities or of habitat within 

 the limits of the same zoological region. 



By means of these four considerations, we shall have exhaust- 

 ed all that experience and analogy offer as regards theory ; and it 

 will remain for us, 



Fifthly, to resort to artificial method to finish conditionally, 

 with all proper reservations, this great work of reform. 



Here we shall be helped in the choice of types by the aid of 

 consideration and combination ; 1st, of the general form regarded 

 as the generatrix of the modifications which will constitute sim- 

 ple varieties ; 2dly, of the pallial and muscular impressions, and 

 of the ligament ; 3dly, of the general system of structure of the 

 appendices of the shell, whether null, pliciform^, nodulous, spi- 

 nous, symphytiote or not, (these ingenious divisions are due to 

 Mr. Lea ;) 4thly, of the essential and constant character of the 

 structure of the hinge ; 5thly, of the thickness of the shell ; 

 6thly and finally, of the general system of coloring ; for we must, 

 as Mr. Lea has done in many cases, deny specific value to epider- 

 mal coloring, and still more to rays so variable in form, in mag- 

 nitude, and in the different ages, and especially to the superficial 

 coloring of the nacre. 



