Mr. Barnes on the Genera Unio and Alasmedonta. 115 
from its parents, will be again produced, and so on, in an 
endless succession of innumerable varieties. The admis- 
sion of M. Lamarck’s supposition would confirm the thought 
which has frequently and very forcibly struck us, that, prop- 
erly speaking, there is but one species of the whole genus ; 
and perhaps of the whole family. Bat this subject is cov- 
ered with a veil of obscurity. There is yet wanting a series 
of minute and well-directed observations on the hab- 
its and manners of this interesting tribe of molluscous Bi- 
valves. In the mean time we must follow our guides at the 
hazard of being sometimes misled. : 
Brugiére established the genus Unio, but his original ob- 
servation on this subject we have not been able to find. The 
word signifies a pearl, because “ many of them produce ye- 
ry fine pearls,”* and nearly all of them have a pearly inside 
called naker or mother-of-pearl. Pliny in his Natural Hist. 
Lib. IX, Cap. 35, entitled Quomodo et ubi inveniuntur, 
margaritae, uses the word, gives the reason ofits derivation, 
and makes it constantly masculine. In this he is followed 
our countryman Mr. Say who makes it always of the 
masculine gender except in that species for which he gives 
credit to M. Le Sueur. Why the celebrated and accurate 
M. Lamarck, has chosen to make it femimine, we cannot 
evenconjecture. Order ef description. No certain order has 
hitherto been adopted by Naturalists in theirdescription of Bi- 
valves. The descriptions both of M. Lamarck and Mr. Sa 
are without a definite method. ‘Though they generally be- 
gin with the outline of the shell, yet they throw together pro- 
miscuously the other parts, both mternal and external. | 
propose to reduce this subject to order in the following 
manner. In examining a bivalve, the first thing that strikes 
the eye of the observer is the outstde, the second is the in- 
side. Hence the description will be divided naturally into 
two parts, the External and the Internal. As it is by the 
interior that we determine the genera of the Naiades, as 
* M. Lamarck. 
