TRIBE NAYADES. 183 
transverse, inequilateral, very thick, posteriorly rounded, anteriorly 
attenuated and obtusely subangulated ; the beaks very broad, promi- 
nent and incurved ; anterior slope very wide and nearly flat, with 
a curved widening furrow, anterior ventral edge incurved; cuticle 
yellowish brown with numerous hair-like rays, nacre white : 
cardinal teeth thick, lateral short and straight. 2..21.—R. 
Tennessee. 
U. Trianeuiaris. Barnes. Sil. J. 6. t. 13. f. 17.—Hat.— 
Hild.— Say. Am. C. t. 4.—Formosus. Lea. on U. 1. (male) 
p. 121.—Tr. Am. Ph. 4. t. 17. f. 42.—Cuneatus. Swain. in 
Phil. Mag. 1823. Triangular, very convex, gaping at the extre- 
mities, the dorsal line forming a rectangle at the beaks, the ventral 
edge simplyarcuated and subincurved anteriorly, extremitiesrounded ; 
anterior slope very much compressed ; beaks broad, pale yellowish 
olivaceous with green rays of interrupted lines and spots which are 
sometimes sagittate (disappearing near the margin in old specimens) ; 
longitudinal slightly elevated lines cross the transverse wrinkles. 
1} ..13.—North America. 
U. Execans. Lea. on U. 1. p. 93. t. 19. f.13.—TZ'r. Am. Ph. 
4. t.9.f. 13. Subtriangular, transverse, subequilateral, thick but 
thinner and acutely angular anteriorly, flattened over the umbones : 
cuticle yellowish green with numerous rays formed of zigzag radiat- 
ing lines, nacre pearly white and irridescent, rarely rose coloured :. 
beaks incurved, flattened and nearly touching: ligament short and 
thick : cardinal teeth large and elevated, the lateral nearly straight. 
13..1,9,.—Ohio. 
U. Donacirormis. Lea. on U. 1. p. 9. (§ Tr. Am. 3.) t. 
4.f.3. Inequilateral, cuneiform, transverse, wrinkled, of moderate 
thickness pointed anteriorly, rounded posteriorly ; cuticle olive with 
green rays; nacre pearly white: beaks anteriorly angulated, not 
decorticated, almost touching: anterior dorsal edge subcarinated : 
cardinal tooth very prominent. 1... 13— Ohio. 
U. Ziczac. Lea. on U.1. p. 54.t.12.f.19.—T'r. Am. Ph. 3. 
J. 19.—at. Ovate, inequilateral, ventricose, thickish, not at all 
angulated : beaks rather prominent, often eroded ; ligament short 
and thick, cuticle yellowish covered with rays composed of fine zigzag 
green lines, nacre pearly white ; cardinal teeth large and erect, the 
lateral curved. ,3,-. 1$.—Ohio. 
U. Hereropvon. Lea. on U. 1. p. 42.t. 8. f. 11.—TZr. Am. 
Ph. 3.t.8.f.11. Rhomboidal-ovate, inequilateral, ventricose, thin, 
transverse, rounded posteriorly, anteriorly angulated and broader : 
dorsal edge straightish; cuticle wrinkled, greenish brown with 
oblique obscure rays, nacre white; beaks prominent : cardinal teeth 
compressed and white, the lateral slightly curved, the double tooth 
in the right valve. =%,..14.—Pennsylvania. 
U.Penrrus: Conr. N. FF. W.S. p. 3320 t1 Saft. Tri- 
