TRIBE MYAIUA. 21 



pellucid, smooth ; pearly within, and the hinge with an oblique, 

 shallow, spoon-shaped tooth. |..l. — Brazil. 2 



A. Rugosa. Lam, 7. Shape square with the angles rounded 

 off, convex, thin, pellucid, obliquely wrinkled. — St. Domingo. 

 Rather smaller titan the last t and the teeth less isolated. 



A. Imperfecta, ham. 8. Ovate, scarcely equivalve, the ante- 



2 Deshayes makes this species the type of Periploma Schu- 

 macher, which is thus characterised. Oval, very inequivalve and 

 extremely inequilateral, one side short, subtruncated and but 

 slightly gaping : hinge in each valve with a straight and oblique 

 spoon-like tooth, forming with the upper margin a deep slope, in 

 which is locked a small triangular shelly substance, which adheres 

 by a portion of the ligament : muscular impression of the trun- 

 cated end very straight and marginal, of the other extremity very 

 small and rounded. 



P. Lenticularis. Sow. Z. P. 1834. Elliptical, lenticular, 

 aequivalve, white, dull, thin : epidermis very thin : posterior dorsal 

 edge sinuated and forming an angle with the posterior margin. 



"i i • 'io ' — ^ e °f Muerta. 



P. Argentaria. Com: Journ. A. N. S. 7. p. 238. t. 18. f. 8. 

 Elliptical, anterior side short, subangulated, slightly reflected, ex- 

 tremity nearly direct, truncated ; beaks rather prominent : apophysis 

 produced, very oblique ; nacre silvery. 2. — U. California. 



P. Planiuscula. Sow. Z. P. 1«34. Oblong, flattish, insequi- 

 valve, whitish, dull, thinnish : the posterior side short, somewhat 

 wrinkled, subtruncated and somewhat sloping at its edge : dorsal 

 edge straightish : epidermis thin and palish. lj-..2-|. — W. Co- 

 lumbia. 



A. Myalis, Lam. 9, belongs to Leach's genus 



THRACIA. 



Oval oblong, subequilateral, inequivalve, slightly gaping at each 

 extremity . Hinge consisting of a more or less prominent hori- 

 zontal spoon-shaped tooth, which receives the ligament. Palleal 

 scar deeply impressed. 



T. Pubescens.* Blainv. — Mya Declivis. part. D. p. 43. — 

 W. t. 2.f 4.— Wood. G. C. t. 18. /. 2.— Anatina P. Turt. B. 

 (young) t. A.f. 3. — A. Myalis. Lam. §.— Kiener Thracia, t. 2. 

 f. 2. Ovate oblong, subventricose, inequivalve, subequilateral, ashy 

 white, posteriorly rounded, anteriorly subtruncated and angulated : 

 hinge furnished with a spoon-shaped process in each valve, whose 

 cavity is divided by a single tooth or ridge. 2|-..3^ — IV. Europe. 

 T. Plicata. Desk. E. vol. 3. p. 1039.— Kiener Thracia, p. 6. 

 t. 2. f. 3. (bad) — Reeve, t. 35. f. 2. Ovate-oblong, transverse, 



