CONCHIFERA. 415 
Famizy II.—Avyicutipm. Wing-shells, 
Shell inequivalve, very oblique, resting on the smaller (right) 
valve, and attached by a byssus; a 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, 
their margins fringed ; foot small, spinning a byssus; gills two 
on each side, crescent-shaped, entirely free (Desh.) or united to 
each other posteriorly, and to the mantle (as in the Oyster, and 
not as in Pecten). 
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. 
Fig. 217. ir 2 
AvicuLA (Klein), Bruguiere. 
Etymology, avicula, a little bird. 
Type, A. hirundo, Pl. 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. 
