TRIBE NAYADES. 177 



arcuated ; thin, indistinctly folded on the anterior parts, beaks 

 finely undulated at the tip : cuticle finely wrinkled, yellowish 

 brown with numerous indistinct greenish rays which anteriorly are 

 disposed to be clouded ; nacre white posteriorly, anteriorly irri- 

 descent, brownish purple in the cavity : cardinal teeth small and 

 erect, the lateral ill defined. ^ . . l^. 



U. AcuTissiMus. Lea. on U. 1. p. 99. t. 10. f. 18. SmaU, 

 narrow elliptical, inequilateral, transverse, very thin, acutely angu- 

 lated anteriorly, with undulations diverging from the umbonal 

 slope ; beaks slightly prominent ; ligament linear ; cuticle yellow 

 smooth and shining, nacre salmon coloured ; cardinal teeth small, 

 the lateral long and straight. J . . l-p'j. — R. Alabama. 



U. MuRCHisoNiANUS. Lea. Trans. Ph. Am. 5. t. 2>. f. 6. 

 — U. DouGLASi^. Gray. Narrow-elliptical, transverse, thin- 

 nish, angular and produced anteriorly, ligamental edge straightish, 

 ventral edge incurved, beaks and umbones plicated, umbonal slope 

 subcarinated and rough with the angles of the folds ; ligament 

 narrow and yellow : cuticle dark green, nacre splendidly pearly, 

 shghtly salmon coloured and irridescent : cardinal teeth double in 

 both valves, the lateral straight. | . . 1^^^. — China. 



U. Grayanus. Lea. Transi. Am. Ph. 5. t. 9. J". 26. 

 Lanceolate, very transverse, thin, rounded posteriorly, very 

 produced and very acutely angular anteriorly, plicate about the 

 beaks and the anterior .side, flattened on the sides, ventral edge 

 slightly incurved in the middle and little arcuated ; umbonal 

 slope ridged, green, the beaks depressed : cuticle yellowish with 

 obsolete rays, nacre pearly and irridescent : cardinal teeth double 

 in both valves and erect, the lateral very long thin and nearly 

 straight, ^. . 3^^. — China. 



U. PoNDEROsus. Lea. — Mya P. D. p. 51. — Mya Crassa. 

 Wood. G. C. t. 20. /. 21.— TF. t. 2. /. 28. Ovate, ponderous, 

 narrow anteriorly and truncated towards the cartilage slope, the 

 beaks decorticated ; cuticle dark brown, longitudinally wrinkled in 

 tM'o different directions : primary tooth very strong shutting into 

 a corresponding cavity. 3^ . . 6. — China. 



* * Nodulous. 



U. Lachrymosus. Lea. on U. 1. f. 14. t. 6. f. 8. Sub- 

 quadrangular transverse, rather depressed and rounded posteriorly, 

 thick, anteriorly angular above and below : beaks recurved, approxi- 

 mate ; not decorticated, covered with delicate raised points which 

 radiate and enlarge in tear -like drops, leaving however untouched 

 an oblique channel running anteriorly to the ventral margin ; 

 cuticle yellowish green, very smooth, posteriorly slightly rayed, 

 nacre pearly white and silvery. It^ • • ^z- — Ohio. 



