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. — Georyia in 

 Aynei-ica. 



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

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

 in J. A. N. S. Pldl. 1. — Barnes in Sil. 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 obliq' ely dorsally and ante- 

 riorly, beaks at the posterior third, rather &' lall but elevated : cuti- 

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

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

 looking backwards. 1-^^ . . 2. — Massuchusets, ^c. 



U. Rugosa, — Alasmodonta R, Barnes in Sil. J. G. t. 13./. 

 21. — Eat. ^- Hild. — U. Abdxjcta. 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 ciiocolate : teeth large and 

 elevated vpith a fold behind. 2 . . 3 J^. — N. America. 



Section B. — Smooth. 



U. Deltoidea.— Marg. D. Lea. on U. 2. f. 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. -f^ . . If. — Ohio. 



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

 (I^owM^.) A. ScuLPTiLis. Say. — Mya Undulata. Wood. S. t. 

 l.y. 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. 

 If . . 2. — MaKsachusets, ^'c. 



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

 Ph. 5, t. \7 . f. 50. Transverse, oblong, anteriorly longer and 



p 2 



