TRIBE NAYADES. 211 



large, the umbonal slope acutely carinate, the anterior slope very- 

 much flattened, having an indistinct furrow along the edge of the 

 carina, cuticle dark green, with numerous green rays over the whole 

 surface, nacre bluish : teeth lamellar and irregular. — Georgia in 

 America . 



U. Marginata. — Marg. M. Lea. Tr. Am. Ph. 6. — Varicosa. 

 Lam. — Mya Rugulosa. W. S.f. 1 . — Alasmodonta M. Say. 

 in J. A. N. S. Phil. 1. — Barnes in Si I. J. 6. — Gould. Mass. p. 

 116.^.77. Transversely ovate, thin, inequilateral, gaping ante- 

 riorly, posteriorly narrow, rounded at both extremities above and 

 below ; ligamental edge scarcely sloping, ventral subincurved in the 

 middle ; umbonal ridge elevated and well defined, anterior slope 

 with coarse rounded wrinkles running obliquely dorsally and ante- 

 riorly, beaks at the posterior third, rather small but elevated : cuti- 

 cle olivaceous, shining, obscurely rayed with broken lines : nacre 

 bluish white, with a chalky white margin ; tooth small, compressed, 

 looking backwards, lyo • • 2. — Massuchusets, fyc. 



U. Rugosa. — Alasmodonta R. Barnes in Sil. J. 6. t. 13. f. 

 21. — Eat. fy Uild. — U. Abducta. Say. Oblong, thick, rounded 

 at both ends, posterior side not quite so long nor broad as ante- 

 rior, ligamental edge nearly straight, not sloping, ventral edge 

 little curved, anterior slope with deep divergent folds, the ridge 

 little elevated, the folds indenting the edge; cuticle chesnut with a 

 silky lustre; nacre flesh colour, becoming at the margin pearly 

 white, with a narrow margin of dark chocolate : teeth large and 

 elevated with a fold behind. 2 . . 3 T 7 ^. — N. America. 



Section B. — Smooth. 



U. Deltoidea. — Marg. D. Lea. on U. 2. p. 43. t. 13. /. 38. 

 Ovate, inequilateral, compressed, thin, obliquely rounded posteriorly, 

 obtuse anteriorly, the anterior ventral margin incurved ; beaks pro- 

 minent, undulated at the tip and devoid of the green rays which 

 elsewhere adorn the yellow cuticle : cardinal teeth erect, the lateral 

 obsolete : nacre white and irridescent. -^ . . 1J. — Ohio. 



U. Undulata. — Alasmodonta U. Say. Am. C. t. 16. — 

 {Young.) A. Sculptilis. %.-Mya Undulata. Wood.S.t. 

 l.f.5. — U. Glabratus. Sow. Transversely ovate, inequilateral, 

 strong, much inflated, widely gaping, anteriorly attenuated and 

 angulated, posteriorly and at the ventral edge rounded ; beaks 

 tumid elevated and undulated : cuticle shining dark green, with 

 obscure linear rays alternately yellow and dark, nacre white ; one 

 cardinal tooth in each valve supported by a strong internal rib. 

 1 J- . . 2. — Massacliusets, §c. 



U. Swananoensis. — Marg. Raveneliana. Lea. Trans. Am. 

 Ph.o. t. 17. f. 50. Transverse, oblong, anteriorly longer and 



p 2 



