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CATALOGUE OF SHELLS. 
19. U. Subrotundus. 
20. U. Truncatus Say. 
Massachussets. 
21. U. Asperimus. Von de Busch. 
Ohio. 
22. U. Radiatus. 
Ohio. 
23. U. Balanus. Mr. Malherbe. 
France. 
24. U. Ovalis. 
25. U.-—— 
202150; 
Ayacan. 
T. W. Russel. 
A singularly interesting species exhibiting in a very marked degree 
the wing like expansion of the valves over the ligament which 
is observable in the American U. Alatus. The wing being 
formed by a shelly fold, the result of the expansion of either 
valve over the ligament, which is as it were rendered internal, 
and there being no joint or breach of continuity in the shelly 
layer or fold, the extent to which the animal can open its valves 
is inconsiderable, and determined by the elasticity of the shelly 
wing. 
27. U. Multiplicatus. 
Ohio. 
28. U. Alatus Say. 
29. U. Tuberculatus. 
80. U. Subovatus. 
Fox River Mich Ter. 
