TRIBE NAYADES. 291 
smooth, and prominent: within pearly, with purple stains beneath 
the hinge, 34. 
A. Guauca. Lam. 13.—Del. t. 13. f. 3.—Valen. in Hum. 
Zool. Obovate, tumid, thin, fragile, obsoletely rayed, with green 
on an olive ground, compressed and winged anteriorly, posteriorly 
very short, narrower and obliquely rounded at the ventral margin. 
33.— Acapulco in America. 
A. Paraconica. Lam. 15.—. t. 203. f. 1. Obovate, anteriorly 
angulated, compressed and carinated on the anterior slope, yellowish 
or brownish green, with concentric transverse strize and grooves ; 
beaks slightly prominent; sides rounded ; ventral edge simply 
arcuated, not incurved ; sinus of posterior hinge margin very short, 
incurved and angulated at its extremity. 3.—Patagonia and La 
Plata. 
A. Ostusa. Spia. Test. Braz. t. 22. f. 3, 4. Oblong-ovate, 
obtuse, very slightly dilated anteriorly, solid, transversely grooved : 
dorsal edge straightish, winged and angulated at each extremity ; 
beaks prominent and decorticated: umbones flattened and smoothish, 
edges thin and sharp: cuticle olive with obsolete radiating blackish 
lines, (in the young with additional angularly flexuous transverse 
ones ;) nacre bluish and irridescent. 13..2.—Brazil. 
A. Srrionos. D’Orbig. in Mag. de Z. 1835. Obovate, 
compressed, thin, olive coloured, very inequilateral, the posterior 
side narrowed, ligamental edge elevated and angulated at the extre- 
mity, the posterior dorsal edge short and incurved, posterior edge 
obliquely sloping to meet the much arcuated ventral margin ; nacre 
greenish. 23..4.— Bolivar. 
A. Latromareinata. Lea, Am. Ph. Tr. 1837. t. 12. f. 34.— 
D Orb.—A. Travezius? Spix.—A. Rorunpvus? Spia. Trans- 
versely obovate, inflated, thick, posteriorly very short and narrow, 
anteriorly dilated and obtuse, dorsal line little curved : cuticle reddish 
brown, finely wrinkled ; nacre pearly white and irridescent, with a 
broad horn-coloured border at the rounded ventral edge; sinus long 
and pointed to the interior of the disc. 2}..33.—. Parana, 8S. 
America. 
A. Sprxur. D’ Orb. in Mag. de Z. 1835.—A, Trapezium. 
Spix. Braz. t. 20. f. 1. Ovate-trapeziform, ventricose, anteriorly 
dilated and rounded, posteriorly much narrower, thick, trans- 
versely striated ; dorsal line somewhat curved, the anterior slope 
elevated and obtusely angulated, posteriorly terminated by an acute 
angle ; margins attenuated and acute : beaks prominent, inflexed, 
decorticated ; umbones inflated ; hinge-lamina somewhat curved 
and terminated anteriorly by an angular sinus: cuticle olive green, 
nacre smooth, bluish-white irridescent. 2..21.—Brazil. 
A. Porcirer. Gray. Z. P. 1834. Ovate, subventricose, thick, 
