THE FAMILY OP NAIADES. 119 



1. Plicate shells.* S. Spinous shells.^ 



2. Nodulous shells.f 4. Smooth shells.^ 



Each of these subdivisions may be again separated, according to the 

 form of their outline, thus : 



1. Quadrate/ 6. Subrotund/ 



2. Triangular.'' 7. Wide.^ 



3. Oblique.' 8. Obovate.'' 



4. Oval.'^ 9. Arcuate.' 



5. Oblong." 



The shell is supposed to be lying on its side with the ligament fur- 

 thest removed from the observer, and the beak to the right of it. The 

 base will of course be nearest to him, and the anterior margin to his 

 right, while the posterior margin will be to the left. This is my mode 

 of arranging my whole cabinet, which contains over 2100 specimens 

 of this family, each differing in some character or locality. 



In attempting to make a complete synopsis of the Naiades, much la- 

 bour has necessarily been expended. I do not present this as a perfect 

 work, but it has been made as much so as the opportunities in my pos- 

 session permitted. Errors may have arisen from two sources: first, 

 default of judgment; second, from accident, owing to the mass of re- 

 search necessary to accomplish the object, considering the crude state 



* As Unio plicatus. Lesueur. 

 t As Unio pustulosus. Lea. 



% As Unio spinosus. Lea. 

 § As Unio complanatus. (U. purpureas. Say.) 



No regard of course is paid in this division to the folds or undulations of the beaks, as all 

 the species are more or less disposed to this character. 



* As U. asperrimus. Lea. 



** As U. triangularis. Barnes. 



'^ As U. clavatus. Lam. 



^ As U. crassus. Say. 



* As U. complanatus. Solander. 

 f As U. circulus. Lea. 



g As U. rectus. Lam. 



h As U. modioliformis. Lea. 



> As M. margaritifera. (Al. arcuata. Barnes.) 



