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 . . 2^. — R. 

 Tennessee. 



U. Triangularis. Barnes. Sil. J. 6. t. 13. f. 17. — Eat. — 

 Hild. — Saij. Am. C. t. 4. — Formosus. Lea. on U. 1. {nude) 

 p. 121. — 2V. A7)i. Ph. 4. t. 17. f. 42. — Cuneatus. Swain, in 

 Phil. 3Ia(). 1823. Triangular, very convex, gaping at the extre- 

 mities, the dorsal line forming a rectangle at the beaks, the ventral 

 edge simply arcuated and subincurved anteriorly, extremities rounded; 

 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. 

 \^ . . If. — North America. 



U. Elegans. Lea. on U. I. p. 93. t. 19./. 13.— Tr. 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. 



I|..l-i^f6^0/«0. 



U. DoNACiroRMis. Lea. on U. \. p. 9, (cj* Tr. Am. 3.) t. 

 A.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 . . IJ — Ohio. 



U. ZiczAC. Lea. on U. I. p. 54. th2.f. 19.— Tr. Am. Ph. 3. 

 f. 19. — Eat. 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, /q . . \\. — Ohio. 



U. Heterodon. Lea. on U. \. p. 42. t. S.f. 11. — Tr. Am. 

 Ph. 3. t. S.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, -j^^^ . . 1^. — Pennsylvania. 



U. Penitus. Com: N. F. W. S. p. 33. t. 5./. 1. Tri- 



