46 GENERA OF SFIELLS. 



AvicuLA. Shell inequivalve, fragile, rather 

 smooth, base transverse, straight ; extremities 

 produced, the anterior caudiform, or tail-like ;. 

 a notch in the left valve. Hinge linear, uni- 

 dentate, beneath the beaks ; area of the liga- 

 ment marginal, narrow, channelled, not trav- 

 ersed by the byssus. 



When the valves are spread open, the shell has a 

 rude resemblance to the wing of a bird, whence its 

 name. They are sea shells, generally smooth exter- 

 nally, thin, and pearly A^nthin. Their beaks are oblique, 

 small, and not prominent. 



A. macroptera, lotorinm. semisagitta, heteroptenij fal- 

 cata, crocea, Terentina, allantira, squamulosa. papiHonacea, 

 physoides, virens, trigonata, pbalœnacca, hirudo. 



Meleagrina. Shell subequivalve, rounded, 

 quadrate, scaly externally ; inferior cardinal 

 margin straight, not caudate anteriorly ; a 

 sinus for the byssus at the posterior base of 

 the valves ; left valve at this part narrow and 

 emarginate. Hinge linear and toothless ; area 

 of the ligament marginal, elongate, subexte- 

 rior, dilated in the middle. 



The meleagrina is distinguished from avicula by the 

 form of the shell, which is nearly cqui valve, by never 

 liaving the tail nor cardinal teeth, and by the widening 

 of the ligaraental facet at the middle part. The aper- 

 ture for the byssus also occasions a callous, re-entering 

 angle on each vah'^e. It often contains true pearls. 



M. margaritifera, albina. 



