TRIBE NAYADES. 209 



reddish brown and wrinkled ; cardinal teeth small, lateral thin and 

 lamellar. 1-^ . . 2^-.— Mexico. 



U. Modioliformis. Lea. Tr. Am. Ph. 5. t. 13. f. 40. — 

 U. Delumbis. Conr. Elongated-ovate, very thin, inflated, trans- 

 verse, rounded at both ends, very narrow and short posteriorly : 

 dilated anteriorly ; beaks small, ligament rather long and thin ; 

 cuticle brown and shining, rays indistinct, nacre slightly purple, 

 very irridescent : cardinal teeth small, compressed, the lateral long 

 and curved. 1^ . . 2 t 7 q. — S. Carolina. 



U. Tenerus. Rave. — Tenebrosus. Conr. F. W. S. p. 42. 

 t. 7-f. 1. Elliptical, ventricose, thickish posteriorly, beaks pro- 

 minent, anteriorly dilated, the edge descending oblique and recti- 

 linear from the ligament, and subtruncated at the extremity, ventral 

 edge simple and little rounded ; cuticle olivaceous with linear rays: 

 the margins dark brown, nacre irridescent bluish or pink, teeth 

 moderately thick. — Virginia. 



U. Tappanianus. Lea. on U. 2. p. 62. t. 17. /• 55. Ovate, 

 rather inflated, inequilateral, thin, anteriorly dilated, ventral margin 

 subincurved, umbonal slope rounded and inflated ; beaks somewhat 

 prominent, with double undulations, ligament slender and shortish; 

 cuticle yellowish brown, with dark rays and the border dark ochre- 

 brown, nacre somewhat salmon-coloured : cardinal teeth compressed 

 and curved, lateral thin. 1 t X q . . 2. — Hollidaysburg. 



U. Contradens. Lea. on U. 2. p. 75. t. 18. f. 58. Ovate, 

 rather inflated, inequilateral, thin, smooth, ventrally and posteriorly 

 rounded, obtusely angulated anteriorly, and the dorsal edge elevated; 

 beaks rather prominent and undulated, ligament thin and longish : 

 cuticle yellowish green becoming green on the anterior slope, nacre 

 white and irridescent : cardinal teeth linear, the double one in the 

 right valve, the lateral thin and somewhat curved, 1-jq • • ^To* 



U. Stramineus. Conr. in Sil. J. 25. t. 1. f. 6. Suboval, 

 convex, anteriorly longer, wider and rounded above and below, ven- 

 tral edge incurved in the middle, beaks slightly prominent, with 

 irregular undulations, umbones convex ; concentric lines remarkably 

 coarse and prominent : cuticle straw colour, rayed only on the ante- 

 rior slope, nacre pearly white and irridescent; cardinal teeth double 

 in both valves and subcompressed. 2\. — R. Alabama. 



ttttttttt Arcuated. 



U. Crassissimus. Fer. — Sinuata. Lam. 1. — Desk, {not 

 Pfeif.nor Blainv.)—Drap. p. 132. t. 10./. 17. to 19. — E. t. 248. 

 f. 1. — Rossm.f. 195. Ovate-oblong, ponderous, thick, dark brown, 

 breadth about twice the length, very inequilateral, posteriorly 

 rounded, anteriorly obliquely and obtusely subtruncated, ventral 

 edge retuse, dorsal arcuated : cardinal teeth thick, the lateral 



