PART II. 



ATRACHI A OSTRACID^. 



387 



umbones lateral or sublateral ; byssus protruded near 

 the umbones. 

 Margarita Leach, (18 17-) Shell more or less rounded; 

 auricles small ; umbones sub-lateral ; the hinge mar- 

 gin straight, but not lengthened ; teeth small, tuber- 

 culated, nearly obsolete, {fig. 126. e.) 

 crocata. Zool. 111. ii. pi. 55 



Avicula Lam. Shell obliquely oval ; the hinge margin 



very much lengthened, and forming unequal auricles; 



teeth tuberculous, nearly or quite obsolete, {fig. 121, 



b, c, d.) 



A. heteroptera. Sow. Gen. f. 1. aculeata. lb. f. 2. 

 Pinna. Shell equivalve, w^edge-shaped or triangular ; 



umbones terminal ; the other extremity widely gaping ; 



hinge margin straight ; teeth none ; anterior margin 



slightly gaping.* {fig. 126. /, g.) 

 P. serrata. Sow. Gen. 



Family 4. OSTRACTDiE. Oysters. 



Animal sedentary, generally affixed by their under valve ; 

 shell foliaceous, rarely pearly, {fig. 129-) 



Pecten Linn. Shell regular, depressed, nearly orbicular, 

 auriculated ; umbones central ; ligament small ; in- 

 ternal hinge margin short ; teeth straight. 



129 



* This genus, by coining close to Mytilus completes the circle of this 

 family. 



c c 2 



