50 GENERA OF SHELLS. 



The spondyli are distinguished from thq ostreee by 

 the cardinal teeth ; they are generally covered with 

 spines, which are occasionally very large, disposed in 

 rows or radiating strise or ribs. The lower valve is 

 always largest and most convex, and is terminated at 

 the beak by a kind of flattened, inclined, triangular 

 facet, which appears as if cut with a knife, and increas- 

 ing by age. 



S. geedaropus, Americanus, arachnoides, candidus, multi- 

 lamellatus, costatus, variegatus, longi-spina, regius, avicu- 

 laris, coccineus, crassi-squama, spathuliferus, ducalis, longi- 

 tudinalis, microlepos, croceus, aurantius, radians, zonalis, 

 violascens. 



PODOPSIS. Shell inequivalve, subregular, 

 adhering by the inferior beak, not auriculated, 

 the lower valve largest, most convex, and the 

 beak most produced ; hinge toothless ; liga- 

 ment interior. 



Fossil. 



OSTRACEA. 



Ligament internal or s emi- internal ; shell irregular, foliated } 

 sometimes very thin. 



The ostracea are seldom auriculated at base, and still 

 more rarely radiated externally ; in many species the 

 shell is fixed to marine bodies by the lower valve, 

 which is always the largest. 



(.) Ligament semi-internal ; shell foliated, and often very 

 thick. 



GRYPHJEA. Shell free, inequivalve ; inferior 

 valve large, concave, terminated by a project- 

 ing beak, curved spirally inwards; superior 



