TRIBE NAYADES. 221 



smooth, and prominent : within pearly, with purple stains beneath 

 the hinge. 3£. 



A. Glauca. 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. 

 3£. — Acapulco in America. 



A. Patagonica. Lam. \5.—E. t. 203. f. 1. Obovate, anteriorly 

 angulated, compressed and carinated on the anterior slope, yellowish 

 or brownish green, with concentric transverse striae 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. Obtusa. Spix. 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. 1^ . . 2. — Brazil. 



A. Sirionos. 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. 2f . . 4. — Bolivar. 



A. Latomarginata. Lea. Am. Ph. Tr. 1837. t. 12. /. 34. — 

 D'Orb. — A. Trapezius? Spix. — A. Rotundus 1 Spix. 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. 1\ . . 3£. — R. Parana, S. 

 America. 



A. Spixii. 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 . . 2^. — Brazil. 



A. Porcifer. Gray. Z. P. 1834. Ovate, subventricose, thick, 



