CONCHOLOGIA CESTRICA. 85 
Station, Susquehanna River and tributaries; abun- 
dant. 
Lingual dentition of A. dissimilis, — |Troschel.] 
Fig, 180, 

Class ACEPHALA. 
In this class the shell is bivalve, the valves being 
united at the back, by a hinge, as in the common creek 
mussel. 
Animal, acephalous; the mouth is a simple aperture, 
without teeth, or jaws, and often takes the form of a 
tube, reaching even beyond the shell, and accompanied 
by another —the vent; which are simple, or fringed. 
Respiration is performed by two, or four, broad leaves, 
or gills, which hang down on the sides of the body, 
within the mantle, and are embraced by the shell, like 
the leaves of a book by the cover. The animal is either 
free, as the mussel, or attached, as the oyster. 
Their food is only such animal, and vegetable, matters, 
as the water may chance to carry into their suctorial 
mouths. 
They are bisexual, as the mussel; or hermaphrodite, 
and capable of self-impregnation, as the oyster. ° 
Order BRANCHIFERA. 
Characters the same as those of the class. 
8 
