276 
vmomjyM, 
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. variahilis^ each with several 
varieties ; yet when you consider the characters by 
which 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 l| 
before named by Say, and Unio is generally kept for |j 
the following genus, we are induced to adopt Say’s i 
name. I 
