TRIBE NAYADKS. 20/ 



very thin and fragile, obliquely rounded posteriorly : the dorsal edge 

 connate and anteriorly rather elevated, ventral edge little rounded 

 and not incurved ; beaks depressed, but pointed ; ligament con- 

 cealed ; cuticle yellow, finely and obliquely rayed with green, nacre 

 bluish white and purple, irridescent ; cardinal tooth a small lobe, 

 lateral teeth acicular and single in both valves. 2± . . 2-1. ~ Ohio. 



U. Bilineatus. Benson. — Symphonota. B. Lea. on U. 1. 

 p. 108. (and Tr. Am. Ph. 4.) t. 11. /. 19. Subelliptical, trans- 

 verse, inequilateral, compressed, very thin, dorsal edge angulated at 

 both ends and anteriorly elevated and connate; ventral edge rounded 

 and entire ; beaks very little elevated, concentrically undulated, 

 with two elevated lines running to the anterior margin ; cuticle glossy 

 greenish yellow and darker on the anterior slope ; nacre slightly sal- 

 mon-coloured ; cardinal teeth lamelliform and double in the right 

 valve only, the lateral straight. t 7 q . . 1 t 3 q. — R. Hoogly, Hindostan. 



U. Corrianus. Lea. Trans. Am. Ph. 5. t. 9.f. 25. Narrow 

 elliptical, very transverse, very thin, very inequilateral, dorsal and 

 ventral edges subparallel straightish and entire, obtusely acuminated 

 anteriorly, beaks scarcely prominent, ligament long and slender : 

 cuticle smooth, rayless, dark brown, nacre pearly white and irrides- 

 cent ; cardinal teeth thin and bladed, the lateral long thin and nearly 

 straight. 1 . . 2^. — India. 



U. Phaseolus. Hild. in Sil. J. 14. p. 283. — Eaton, in 

 Transylv. J. of Med. 1831. — U. Planulatus. Lea. on U. I. p. 

 45. andTr.Am. Ph. 3. Ovate elliptical, transverse, inequilateral, 

 rounded at both ends, peculiarly flattened over the umbones, thick ; 

 beaks slightly prominent and decorticated; ligament little pro- 

 minent ; cuticle wrinkled ; yellowish brown, marked with square 

 spots disposed in rays, nacre bluish white : cardinal tooth small, the 

 lateral large thick and curved. \-fg . . 2-i. — Ohio. 



U. Arcus. Conr. Sil. J. 25. t. 1. f. 8. Narrow-elliptical, 

 thick, ponderous, transverse, posteriorly short and rounded, ante- 

 riorly produced and attenuated, ventral edge little arcuated and in- 

 curved ; dorsal edge forming an arched curve scarcely interrupted 

 by the beaks ; cuticle olive and wrinkled, nacre pearly white ; car- 

 dinal teeth thick, pyramidal and distant from the lateral. 2. — i?. 

 Alabama. North America. 



U. Arctatus. Conr, Sil. J. 25. t. 1. f. 9. Narrow-ellip- 

 tical, elongated, much compressed, slightly contracted from the 

 depressed beaks to the base, anteriorly much produced and suban- 

 gulated at the end, cuticle very dark olive, nacre bluish white : ven- 

 tral edge incurved in the middle, but little arcuated; cardinal teeth 

 disposed to be single in both valves, the lateral compressed and a 

 little prominent. 2. — R. Alabama. 



U. Gibbosus. Barnes Sil. J. 6. t. U. f. 12.— Eat. in Tr. 

 Jour. 1831. — Hild. in Sil. J. 14.— U. Mucronatus. Barnes. — 



