270 AVICULIDA. 
(Aviculacea.) 
The following families are often included in Monomyaria: 
Famity AVICULID &. 
Shell inequivalve, very oblique, resting on the smaller (right) 
valve, and attached by a byssus; epidermis indistinct ; outer 
layer prismatic-cellular, interior nacreous; posterior muscular 
impression large, subcentral, 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 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. The family is mostly 
extinct, and largely represented in palzozoic rocks; there are 
120 recent and over 1000 fossil species. 
SuBFAMILY AVICULINA. 
Ligament attached to the entire external hinge-margin or 
placed in a single shallow groove near the beak and spreading 
over the hinge-area as it extends posteriorly ; anterior muscular 
scar very small. 
Avicuna (Klein), Lamarck, 1799. 
Etym.—Avicula, a little bird. 
Syn.—Pteria, Scopoli, 1777. Anonica, Oken, 1815. 
Distr.—25 sp. Mexico, South Britain, Mediterranean, India, 
Pacific ; 20 fathoms. Fossil, 300 sp. Lower Silurian—; world- 
wide. A. helerapiera, Lam. “(Cxxxi, 61). 4A. crocea, "Lame 
(cxxxi, 62). 
Shell obliquely oval, very inequivalve, eared, the posterior ear 
produced, wing-like ; right ‘valve with a byssal sinus beneath the 
anterior ear ; artilage- -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, subcentral ; anterior (pedal scar) small, umbonal. 
Animal oval, flat; mantle-lobes separated throughout, thick- 
ened and serrated at the margins; body very small, having on 
either side a pair of nearly equal large branchize ; mouth oval, 
rather large ; palpi large, obliquely truncate; byssus large, coarse, 
sometimes consolidated. 
