52 GENERA OF SHELLS. 



Vulsella. Shell longitudinal, subequivalve, 

 irregular, free ; beaks equal. Hinge with a 

 prominent callosity on each valve, depressed 

 above, and exhibiting a conical and obhquely 

 arcuated pit for the ligament. 



The vulsella is allied to the ostreee, but always has 

 the valves nearly equal ; the beaks equal, though some- 

 what separate ; an equal, projecting callus in the inte- 

 rior of each valve under the beaks ; and it is never 

 fixed by its lower valve. They are often found in 

 sponges ; some species gape a little at the posterior 

 side. 



V. lingulata, hians, rugosa, sporigiarum, mytilina, ovata, 

 deperdita. 



(b.) Ligament internal ; shell thin, papi/raceous. 



Placuna. Shell free, subequivalve, irregu- 

 lar. Hinge internal, presenting on one valve 

 two sharp, longitudinal ribs, in form of V ; on 

 the other, two corresponding impressions. 



The valves of the placunae are thin, transparent, and 

 of the same size. They are large, orbicular or subtri- 

 angular, and sometimes triangular. Their texture is 

 foliated. 



P. sella, papyracea, placenta, pectinoides — pertusa, quad- 

 rifasciata. 



Anomia. Shell inequivalve, irregular, oper- 

 culated ; adhering by the operculum ; lower 

 valve pierced, commonly flat, with a hole or 

 notch at the beak ; the other larger, concave, 

 entire ; operculum small, elliptic, osseous, fixed 



