482 HISTORY OF 



H. palliata'? H. fuliginosa ; H. hirsuta ; H. pulchella; H. in- 

 dentata ; H. electrina"! H. labyrinthica. 



1 have found Succinea obliqua upon the islands in the Sus- 

 quehanna, opposite to Washington. The shell presents a 

 strong resemblance to some of the forms of Limnea colum- 

 ella, a species which will probably be detected hereafter in 

 this county, as it occurs in other parts of the state, as in the 

 vicinity of Philac'olphia, and in York county. 



UNI0NID.E. 



This family includes all our large bivalve shells, usually 

 celled mussles. 



Unio is the principal genus, and contains the greatest num- 

 ber of species. It is known by having one or two short, robust 

 teeth at the hinge, and behind them one or two others, which 

 are flat and blade-shaped; the former are called cardinal — the 

 latter lamdlar teeth. 



Alasmodon wants the lamellar teeth, and 



Anodon is without either kind. The following species in- 

 habit the Susquehanna and branches: 



Unio cariosus; shell straw yellow, 3 or 4 inches. U. radia- 

 tus; covered with broad green bands, 4 or 5 inches. U. com- 

 planatus ; compressed, dull brown, inside frequently purple ; 

 young sometimes rayed ; extremely variable in form ; our 

 most common species, 3 inches. U. viridis; a small, fragile, 

 brown or green, rayed species, with the cardinal teeth com- 

 pressed, and very variable ; usual length 1^ inch. 



Alasmodon undulatus; dark brown, rayed, a very robust 

 tootli in each valve, 11 inch. A. marginatus; green, rayed; 

 ^cardinal tooth small and thin ; posterior extremity of the shell 

 truncated ; 2 inches. 



Anodon cataractus; bright green, rayed; delicate, 4 or 5 

 inches. 



Anodon undulatus; dark brown, hinges slightly thickened, 

 having a tendency to form a slight pair of teeth, 2 or 3 inches. 

 JBec Conrad's work for information upon this family ; Binney's 

 on those of the land, and Haldeman's on the frcslnvater uni- 

 Talve species. 



