260 MANUAL OF THE MOLLUSCA. 



FAMILY II. AvicuLiCE. Wing-shells. 



Shell inequivalve, very oblique, resting on tlie 

 smaller (right) valve, and attached by a byssus ; 

 epidermis indistinct : outer layer prismatic-cell alar, 

 (fig. 180) interior nacreous; posterior muscular 

 impression large, sub-central, anterior small, within 

 the umbo ; pallial line, in-egularly dotted j hinge- 

 line straight, elongated ; umboues anterior, eared, 

 the posterior ear wing-like ; cartilage contained in 

 one or several grooves; hinge edentulous, or ob- Fig. iso, Pinna.* 



scurely toothed. 



Animal with the mantle-lobes free, their margins fringed ; foot small, 

 spinning a byssus ; gills 2 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 Vecten). 



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

 rate seas ; there are no living species in northern latitudes, where fossil forms 

 are very numerous. 



AvicuLA (Klein) Bruguiere. 



Etpn. Avicida, a little bird. Type, 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 1 or 2 

 small cardinal teeth, and an elongated posterior tooth, often obsolete ; pos- 

 terior 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 

 4, posterior large in front of the adductor ; adductor composed of 2 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. (Brit. M.) 



Distr. 25 sp. Mexico, S. Brit. Medit. India, Pacific:— 20 fms. 



Fossil, 300 sp. L. Silurian — . World-wide. 



Suh-genera, Meleagrina, Lara. M. margaritifera, V\. XYI. fig. 19. 

 The "pearl-oysters" are less oblique than the other aviculce, and their valves 

 are flatter aud nearly equal ; the posterior pedal impression is blended with 

 that of the great adductor. They are found at Madagascar, Ceylon, Swan 



*• 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. 



