THE FAMILY OF NAIADES. XIX 



After the divisions of Symphynote and Non-Symphynote shells, we have the four 

 conditions in which the outward surface of the shell is found, viz. : — 



1. Plicate. 1 3. Spinous. 3 



2. Nodulous. 2 4. Smooth. 4 



Each of these subdivisions group according to the form of their outline, thus : — 



1. Quadrate. 5 6. Subrotund. 10 



2. Triangular. 6 7. Wide. 11 



3. Oblique. 7 8. Obovate. 12 



4. Oval. 8 9. Arcuate. 13 



5. Oblong. 9 



In regarding these outlines, the shell is supposed to be lying on its side with the liga- 

 ment furthest 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 will be to his right, while the poste- 

 rior margin will be to his left. This is my mode of arranging my whole cabinet, which 

 contains over 4500 specimens of this family, each differing in sex, age, some characteristic, 

 or geographical distribution. 



In attempting to make a complete synopsis of the Naiades, much labor 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 possession permitted. Errors may have arisen from two 

 sources: first, default of judgment; second, from accident, owing to the mass of research 



in the Rio Parana above Corrientes. It resembles an Anodonta, but remains always attached by a byssus. This very 

 remarkable shell has not been as well observed in regard to its habits and anatomy as it deserves to be, and it is 

 mentioned here to draw attention to it, without placing it systematically, as yet, among the Naiades, in which 

 family it probably will be found properly to belong by its anatomical structure. The genera Etheria, Mulleria, 

 Acosteea ? Galatea, Cyclas, and Dreissena, all inhabit, also, fresh water, and have their affinities. 



1 Unio plicatus. Lesueur. 



2 Unio pustulosus. Lea. 



3 Unio spinosus. Lea. 



4 Unio complanatus. Lea. 



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, which is very valuable in discriminating the species. 



5 U. asperrimus. Lea. 



6 U. triangularis. Barnes. 



7 U. clavus. Lam. 



s U. ligamentous. Lam. 

 9 U. complanatus. Lea. 



10 U. circulus. Lea. 



11 U. rectus. Lam. 



12 U. modioliformis. Lea. 



13 M. margaritifera. Lea. 



