CONCHOLOGY. 87 



O. erucella. 0. imbricata, 



O. folium. O. hyotis. 



0. labrella, 0. radiata. 



O. edulis. The eatable Oyster. PI. 16, fig. 5. 



Species orbicular, and not plaited ; ragged, with undu- 

 lated, imbricated scales ; one valve flat and the other con- 

 vex ; variable in size ; outside greenish brown, inside pearlv 

 white, sometimes with a bluish tinge. 



O. Virginica. The Virginian Oyster. 

 Species longitudinal and not plaited. 



O. imbricata. The imbricated Oyster. 

 Species orbicular and plaited. 



O. crista-galli. The Cock's-comb Oyster. 

 Species strongly plaited longitudinally. 



3. Vulsella. The Tweezers. Six species. 



This genus has several characteristics which distinguish 

 it from the Ostrea ; they are free ; the valves and the apices 

 are nearly equal, with a projecting callosity on each valve, 

 depressed underneath, and obliquely arched for the reception 

 of the ligament. 



Shell sub-nacreous, sub-regular, sub-equivalve, inequilat- 

 eral ; upper valve finely granulated, or striated longitudinally 

 from the apex to the margin ; summits anterior, distant, re- 

 curved below ; hinge without teeth ; ligament undivided, 

 thick, inserted in a rounded cavity, made in a slightly pro- 

 jecting callosity on each valve ; muscular impression sub- 

 central, rather large, and two very small ones entirely an- 

 terior. 



V. lingulata. The tongue-like Vulsella. PI. 15, fig. 6. 

 Elongated, depressed, transversely striated ; pale yellow- 

 ish brown, with longitudinal darker stripes. 



