TRIBE NAYADES. 221 



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

 the hinge. 3j. 



A. Glauca. iMm. 13. — Del. t. 13. ,/". 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. 

 3J. — Acapulco in America. 



A. Patagonica. Lam. 15. — E. t. 203. f. I. 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 smootliish, 

 edges thin and sharp: cuticle olive with obsolete radiating blackish 

 lines, (in the young with additional angularly flexuons transverse 

 ones ;) nacre bluish and irridescent. \\ . . 2. — Brazil. 



A. SiRiONos. D'Orhig. 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 ? 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. 2|- . . 3|-. — P. Parana, S. 

 America. 



A. Spixii. B^Orh. 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, subvcntricose, thick, 



