114 Des Moulins on the genera Unio and Anodonta. 
fluviatilis ; ; such as the form of the foot or of the ones of 
the mantle in the Unio radiatus, &c. 
Secondly, we will take for our guide that very saneutii 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 we of this physiological scrutiny. 
. 
Fourthly, ill examine the laws of the geographical distri- 
bution of animals i different classes, and, governed by anal- 
ogy, we will not nit cosmopolitan species, except with the 
greatest reserve ; but e will take equal care not to give too much 
specific importance t a difference of —— or of habitat withit 
the limits of the same zoological regio 
By means of these four parnidarelenal we shall have exhaust 
ed all that experience and analogy offer as wea? theory ; and it 
will remain for us, gee 
Fifthly, to resort to artificial method to fi ch pg nae 
with all proper reservations, this great work of ref 
Here we shall be helped in the choice of one he the aid of 
consideration and combination ; Ist, of the general form regarded 
as the generatrix of the modiGeations 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 nuil, pliciform, nodulous, spt 
nous, symphynote or not, (these ingenious divisions are due to 
Mr. mo Athly, of the essential and constant character of the 
re of the hinge; Sthly, of the thickness of the shell; 
nd finally, of the general system of coloring ; for we must, 
. 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 
eo “ the nacre. 
