276 UNIONID^. 



I believe that these are all varieties of the same 

 species, the size and thickness of the shell depending 

 on the quantity of food and calcareous matter in the 

 pond or river, and the depth of the scars on the 

 thickness of the shell. 



M. Moquin Tandon, in the last work on the 

 French Mollusca, reduced the species inhabiting 

 that country to four, viz., A. cygnea, A. anatina, 

 A. complanata, and A. variabilis, each with several 

 varieties ; yet when you consider the characters by 

 Avhich they are attempted to be divided, it is very 

 evident that, as in England, only a single species is 

 found in France. 



2. Alasmodon Say. (Pearl Muscle.) 



Shell oblong-elongate; hinge with short crested 

 irregular anterior teeth in the right valve, 

 shutting between two in the other valve ; pos- 

 terior lateral teeth small in the young, and 

 wanting in the adult shells ; gills formed of 

 straight tubes, and resembling a kind of trellis- 

 work. 



Found in rapid rivers. 



This genus was established by Say : Dr. Leach 

 named it Damaris, and Dr. Turton has retained for 

 it the generic name of Unio ; but, as this genus was 

 before named by Say, and U7iio is generally kept for 

 the following genus, we are induced to adopt Say's 

 name. 



