CONCHIFEEA. 415 



Family II. — Avictjlid^. Wing-shells. 



Shell inequivalve, very oblique, resting on tlie smaller (right) 

 valve, and attached by a byssus ; epidermis indistinct : outer 

 layer prismatic- cellular (Fig. 217), in- 

 terior nacreous ; posterior muscular im- 

 pression large, sub-central, anterior small, 

 within the umbo ; pallial line, irregularly 

 dotted ; hinge-line straight, elongated ; 

 umbones anterior, eared, the posterior 

 ear wing-like ; cartilage contained in one 

 or several grooves ; hinge edentulous, or 

 obscurely toothed. 



Animal with the mantle-lobes free, Fig. 2u. Pinna* 



their margins fringed ; foot small, spinning a byssus ; gills two 

 on each side, crescent -shaped, entirely free {Desk.) or united to 

 each other posteriorly, and to the mantle (as in the Oyster, and 

 not as in Peden). 



The wing-shells, or pearl-oysters, are natives of tropical and 

 temperate seas ; there are no living species in northern latitudes, 

 where fossil forms are very numerous. 



Ayicula (Klein), Bruguiere. 



Etymology, avicula, a little bird. 



Tijpe, A. hirundo, PI. XVI., Fig. 18. 



Shell obliquely oval, very inequivalve; right valve with a 

 byssal sinus beneath the anterior ear ; cartilage pit single, 

 oblique ; hinge with one or two small cardinal teeth, and an 

 elongated posterior tooth, often obsolete ; posterior muscular 

 impression (adductor and pedal) large, sub-central; anterior 

 (pedal scar) small, umbonal. 



Animal (of meleagrina) with mantle-lobes united at one point 

 by the gills, their margins fringed and furnished with a pendent 

 curtain ; curtains fringed in the branchial region, plain behind ; 

 foot finger-like, grooved; byssus often solid, cylindrical, with 

 an expanded termination ; pedal muscles four, posterior large 

 in front of the adductor; adductor composed of two elements; 

 retractors of the mantle forming a series of dots, and a large 

 spot near the adductor ; lips simple ; palpi truncated ; gills 

 equal, crescentic, united behind the foot. (British Museum.) 



* The cellular structure may be seen with a hand-lens, in the thin margin of the 

 shell, by holding it up to the light ; or on the edges of broken fragments. 



