TRIBE NAYADES; 207 
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. 24..24.—Ohio. 
U. Birrneatus. Benson.—Sympuonora. B. Lea. on U. 1. 
p. 108. (and Tr. Am. Ph. 4.) t. 11. f.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. #7, .. 143,.—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.. 244,.—India. 
U. Puasrotus. Hild. in Sil. J. 14. p. 283.—Eaton. in 
Transylv. J. of Med. 1831.—U. Puanutatus. Lea. on U.1. p. 
45. and Tr. 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. 1-3, .. 24.— 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.—R. 
Alabama. North America. 
U. Arcratus. 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. Gissosus. Barnes Sil. J. 6. t.11. f. 12.—Lat. in Tr. 
Jour. 1831.—Hild. in Sil. J. 14.—U. Mucronatus. Barnes.— 
