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. — 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 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. l.p. 93. t. 19. f. 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. 

 11 - . l-i%. — Ohio. 



U. Donaciformis. Lea. on U. 1. p. 9. ($" 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 . . 1J — Ohio. 



U. Ziczac. Lea. on U. l.p. 54. t. 12. /. 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. 2 % . . lj. — Ohio. 



U. Heterodon. Lea. on U. 1. p. 42. t. S.f. 11. — Tr. 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. T % . . If. —-Pennsylvania. 



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



