TRIBE NAYADES. 187 



U. Cor. Conr. N. F. W. S. p. 28. t. 3. /, 3. Obliquely 

 cordate, thick, with concentric furrows, a slight and broad depres- 

 sion from the beaks to the base ; beaks and umbones very pro- 

 minent ; umbonal slope subangulated, submarginal ; posterior side 

 short, subtruncated, anterior side flattened behind the umbonal 

 slope : cuticle rugose, olivaceous with green interrupted rays (some 

 broad), nacre white and irridescent: teeth much elevated granulated 

 and sulcated. — R. Alabama. 



U. Truncatus. Swain. Z. II. 2d seines. Thick oblong, the 

 umbones close to the posterior margin which is truncated, anterior 

 side produced and cuneiform, ventral edge little arcuated ; very 

 convex, nearly smooth, cuticle fulvous, nacre pearly. If . . 3^. 



U. Decisus. Lea. on U. I. p. 102.— Conr. Mon. U. t.3~f. 2. 

 Very inequilateral, oblique, wedge-shaped, scaleniform and very 

 thick ; anteriorly subacuminated, posteriorly obliquely obtuse and 

 inflated : beaks elevated incurved, nearly terminal, generally 

 decorticated ; ligament rather small ; cuticle yellowish brown some- 

 times with oblique indistinct brown rays, nacre pearly white ; car- 

 dinal teeth smallish, lateral thick. 1-^ . . 3-^. — M. Alabama, 



U. Clavus. Lam. — Conr. Mon. U. p. 5. t. 3.f. 1. — Modio- 

 liformis of Say. (not Lea.) — U. Scalenius. Raf. Ann. de 

 Sc. Phy. 5. t. 81. f. 24, 5. Cuneiform, very oblique ; anterior 

 side produced, compressed, with an oblique furrow rapidly widen- 

 ing from the umbo ; ligamental margin arcuated, very oblique ; 

 umbo elevated, narrowed, inclining posteriorly, beaks curved to a 

 point, terminal ; epidermis wrinkled, olive yellow or brownish 

 with interrupted rays over the umbo, disappearing by age ; inside 

 white, irridescent anteriorly : cardinal tooth in the left valve, nearly 

 parallel with the ligament margin. 1£ . . If. — N. America. 



U. Patulus. Lea. on U. 1. p. 55. (^ Tr. Am. 3.) t. 12. /. 20. 

 Ovate, compressed, wedge-shaped, inequilateral, oblique, transverse, 

 compressed on the umbones, thickish, anteriorly attenuated, sloping 

 from the beaks on both sides, posteriorly obtuse and very short : 

 cuticle yellowish brown, wrinkled, with more or less broad inter- 

 rupted rays, nacre pearly white; cardinal tooth small, the lateral long 

 and slightly curved. If . . 2 T \. — Ohio. 



U. Ravenelianus. Lea. Trans. Am. Ph. 5.t. 3. f. 5. Trans- 

 versely ovate, short and obliquely rounded posteriorly, subangu- 

 lated anteriorly ; ligamental edge straightish, ventral little arcuated ; 

 ligament short and thick ; cuticle dark brown, finely wrinkled, 

 nacre pearly white, partly irridescent ; teeth thick, the cardinal 

 short, the lateral straight. ^ . . 1). — Tennessee. 

 ^ U. Rangianus. Lea. on U. 2. p. 95. (^ Tr. Am. Ph.)t. 18. 

 f. 56. Oblique, somewhat compressed, obovate, very inequilateral, 

 thickish, anteriorly very broad, rounded at both ends, dorsal edge 

 straightish, ventral subincurved in the middle : beaks prominent ; 



