THE FAMILY NAIADES. 7 



After the divisions of Symphynote and Non-Symphynote 

 shells, we have what appears to me four very natural subdi- 

 visions, viz. : 



1. Plicate shells.* 



2. Nodulous shells.t 



3. Spinous shells.^ 



4. Smooth shells.§ 



Each of these subdivisions may be again separated accord- 

 ing to the form of their outline, thus : 



1. Quadrate.* 



2. Triangular.^ 



3. Oblique.<^ 



4. Oval.'i 



5. Oblong.'^ 



6. Subrotund.f 



7. Wide.s 



8. Obovate.h 



9. Arcuate.' 



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



* As Unio plicatus. Lesuenr, 

 ■\ As Unio pustulosus. Lea. 

 i As Unio spinosus. Lea. 



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



No regard of course is paid in this division to the folds or undulations of 

 the bealis, as all the species are more or less disposed to this charactei. 



a As U. asperrimus. Lea. 



^ As U. triangularis. Barnes. 



c As U. clavatus. Lam. 



d As U. crassus. Say. 



e As U. complanatus. Solander. 



^ As U. circulus. Lea. 



S As U. rectus. Lam. 



^ As U. modioliformis. Lea. 



' As M. margaritifera. Al. arcuata. Barnes. 



