260 / M ; \\ [ - ^ J / 01 'T H P 1\1 ?M -US C A . 



FAMILY II. AYICULID^. "Wing-shells. 

 Shell inequivalve, very oblique, resting on the 

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

 epidermis indistinct : outer layer prismatic-cellular, 

 (fig. 180) interior nacreous; posterior muscular 

 impression 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 ob- Fig. 180, 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 Pecteri) . 



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. 



Etym. Amcula, a little bird. Type, A. hirundo, PI. XVI. fig. 18. 



Shell obliquely oval, very inequivalve ; right valve with a byssal sinu 

 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 behinc 

 the foot. (Brit. M.) 



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



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



Sub-genera, Meleagrina, Lam. M. margaritifera, PI. XVI. fig. 19 

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

 are flatter and 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. 



