46 GENERA OF SHELLS. 



AVICULA. Shell iriequivalve, 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 resemhlance to the wing of a bird, whence its 

 name. They are sea shells, generally smooth exter- 

 nally, thin, and pearly within. Their beaks are oblique, 

 small, and not prominent. 



A. macroptera, lotorium, semisagitta, heteroptera, fal- 

 cata, crocea, Terentina, atlantica, squamulosa. papilionacea, 

 physoides, virens, trigonata, phalaenacea, 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 equivalve, by never 

 having the tail nor cardinal teeth, and by the widening 

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

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

 angle on each valve. It often contains true pearls. 



M. margaritifera, albina. 



