TRIBE NAYADES. 219 



straight subparallel to the ventral which is scarcely at all arcuated : 

 beaks flattened and minutely undulated : cuticle very smooth, green 

 and olive green with obsolete rays (the three anterior more distinct) 

 nacre bluish white and irridescent, scars nearly perceptible. 1 ^ . . 3 J-. 

 — Ohio. 



A. Newtonensis. Lea. on U. 2. p. 79. t. 21. f. 66. Narrow 

 elliptical, inflated, thick, ventral edge entire, inequilateral ; beaks 

 somewhat prominent and rather largely undulated at the tip : liga- 

 ment rather short : cuticle dark brown, shining, apparently ray- 

 less : nacre white, sometimes salmon 2 j'g . . 4^. — New Jersey. 



A. MoRTONiANA. Lea. Am. Ph. Tr. 183'7. t. 13. /. 37. — 

 A. Chiquitana. D'0rb.in3Iag.de Z. 1835. Transversely sub- 

 elliptical somewhat inflated, thick, the ventral and ligamental edge 

 nearly parallel and scarcely curved, anteriorly subbiangulated, pos- 

 teriorly very short and narrow : grooved along the umbonal slope ; 

 beaks retuse and scarcely prominent : cuticle intensely brown and 

 slightly wrinkled ; nacre silvery white and irridescent. 1|^ . . 3. — 

 M. Parana, S. Ainerica. 



A. OvATA. Lea. on. U. 2. p. 2. t. 2./. 2. Ovate, somewhat 

 compressed, transverse inequilateral, thinnish, ventral edge rounded; 

 beaks somewhat prominent and granular at tip ; ligament rather 

 short : cuticle greenish brown, wrinkled, apparently ray less ; nacre 

 white or bluish. 2^ . . 4. — Ohw. 



A. Plana. Lea. Am. Ph. Tr. 1837. t. 7. — Declivis. Conr. 

 Ovate-oblong, inequilateral, thick, rather compressed, anteriorly 

 subangular, dorsal line slightly curved, ventral edge but slightly 

 arcuated, posterior side but little narrower ; beaks elevated, granular 

 at the tip ; cuticle smooth, olive to dark green, brighter on the 

 beaks, rays obsolete ; nacre white (rarely salmon near the beaks,) 

 scars perceptible. 3^1q . . 5|. — Louisville, N. America. 



A. Decora. Lea. on U. 2. p. 64. t. 20. f. 63. Elliptical, thin, 

 nearly straight on the dorsal margin, inequilateral, angular an- 

 teriorly ; beaks rather prominent and undulated in a double series at 

 the tip ; ligament rather long and straight ; cuticle smooth and 

 shining, rayed with green beyond the subcentral dark band ; 

 anterior slope dark, with three rays : nacre white just tinged with 

 salmon in the cavity. 2^ . . 3^^. — Ohio. 



A. Gigantea. Lea. on U. 2. p. I. t. 1. Ovate oblong, inflated, 

 thick, posteriorly broad, anteriorly very produced and subcuneiform 

 from the great slope of the ligamental margin ; beaks prominent, 

 granular at tip ; cuticle dark brown, smooth ; ventral edge incurved 

 anteriorly; nacre white and richly pearly. 4^ . . 7^. — Port Gibson.^ 



A. Oregonensis. Jjea. on U. 2. p. 80. t. 21. f. 67. Some- 



' Add likewise the A. Sxjbvexa oJ" Conrad. 



