AVICULA. 95 



Genus I. AVI'CULA*, Klein. PL II. f. 3. 



Body oval : palps or labial appendages large : byssus fibrous, 

 coarse, and thin. 



Shell square or elliptical, often winged or lobed, inequi- 

 valve : upper margin of the lower or left valve notched for 

 the passage of the byssus : beaks placed nearer the anterior or 

 narrower end, but never terminal: hinge furnished with 

 teeth. 



This genus is distinguished by the nacreous and 

 iridescent lining of its shell, which in one species pro- 

 duces the oriental pearl, and in another is extensively 

 used for inlaying and button-making. 



1. Avicula hirun'do f, Linne. 



Mytili's hirundo, Linn. Syst. Nat. p. 1159. A. Tarentina, F. & H. ii. 

 p. 251, pi. slii. f. 1-3, and (animal) pi. S. f. 4. 



Body suboval, yellowish-white mottled with brown : cirri 

 arranged in two rows, numerous and close-set, those in the 

 front or outer row being longer than those in the inner row : 

 foot white. 



Shell elliptical, resembling that of a Pinna, with the 

 addition of an unequal wing-like appendage on each side of 

 the beak, very inequivalve in the young, but less so in the 

 adult state, the upper or right valve being larger than the 

 other, much compressed towards the margins, rather fragile, 

 more or less glossy : sculpture, thin concentric plates or scales, 

 which irregularly imbricate the surface and are sometimes 

 spinous towards the anterior margin, besides superficial micro- 

 scopic and close-set punctures : colour yellowish-brown, with 

 purplish streaks which radiate from the beaks and are fre- 

 quently interrupted or zigzag : margins very thin, rounded in 

 front, more or less deeply incurved on the upper part of eaeh 

 side in consequence of the projection of the wings, straight or 

 nearly so behind : beaks small, sharp, and slightly projecting 

 over the hinge : wings triangular and bluntly pointed ; they 

 are of unequal size, those on the anterior side being small, 



* A little bird. t Swallow. 



