OF THE UNITED STATES. 881 



versely-suborbicular, with small, irregular, inequi- 

 distant. concentric wrinkles, and larger adventitious 

 undulations: base rounded; anterior and posterior 

 edges regularly, equally and very obtusely curved ; 

 beak nearly central ; hinge teeth prominent and dis- 

 tinct ; lateral teeth prominent, white; within im- 

 pressed by the exterior undulations, and bluish- 

 white on the margin and submargin. 



Length nine-twentieths ; breadth eleven-twenti- 

 eths of an inch. 



This species was found by Mr. William Hyde, 

 in a pond near Germantown, in plenty. In compari- 

 son with C. similis, it is thinner, more transparent, 

 not flattened at base, more obtusely rounded each 

 side, and instead of grooves of some degree of regu- 

 larity as in that shell, it is sculptured with irregular 

 wrinkles and waves. Mr. Hyde took fifty young 

 ones out of a single specimen. 



N. B. The genus Cyclas was inserted into this 

 essay inadvertently ; the species, however, are new. 



