Mr. Barnes on the Genera Unio and Alasmodonta. 1 1 5 



children of common parents, and as much like them as they 

 are like each other." If the fecundating fluid, emitted by 

 the male, be received by the female, a variety intermediate 

 between them, will be produced. By a second propaga- 

 tion, by one of the parents and the intermediate, a new va- 

 riety, less different from the former one, than that was 

 from its parents, Avill 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. But 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. 



Brugiere 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 ve- 

 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 of its derivation, 

 and makes it constantly masculine. In this he is followed 

 by our countryman Mr. Sa^^ 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 

 J\'L Lamarck, has chosen to make it feminine, we cannot 

 even conjecture. Order of description. No certain order has 

 hitherto been adopted by Naturalists in their description of Bi- 

 valves. The descri{)tions both o(M. Lamarck and Mr. Say 

 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 internal and external. 1 

 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 outside, 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 Jiaiades, as 



* M. Lamarck. 



