210 AVICULTD^. 



(Aviculacea.) 

 The following families are often included in Monomyaria : 



Family AYICULID^. 



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

 valve, and attached by a byssns ; 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 posteriorl3', 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 mostlj^ 

 extinct, and largely represented in palaeozoic rocks ; there are 

 120 recent and over 1000 fossil species. 



Subfamily A VIC ULIN^. 



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. 



AvicuLA (Klein), Lamarck, 1199. 



Etym. — Avicula, a little bird. 



Syn. — Pteria, Scopoli, 1111. Anonica, Oken, 1815. 



Distr. — 25 sp. Mexico, South Britain, Mediterranean, India, 

 Pacific ; 20 fathoms. Fossil, 300 sp. Lower Silurian— ; world- 

 wide. A. heteroptera, Lam. (cxxxi, 61). A. crocea, Lam. 

 (cxxxi, 62). 



Shell obliquely oval, very inequivalve, eared, the posterior ear 

 produced, wing-like; right valve with a byssal sinus beneath the 

 anterior ear ; cartilage-pit single, obliqvie ; hinge with one or two 

 small cardinal teeth, and an elongated posterior too,th, 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 branchite ; mouth oval, 

 rather large ; palpi large, obliquel}^ truncate ; bj^ssus large, coarse, 

 sometimes consolidated. 



