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. +.. ly 
U. Acutissimus. Lea. on U. haps It 10a. 18s% Small, 
narrow elliptical, inequilateral, sci 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. 3..154,.—. Alabama. 
U. Murcuisonianus. Lea. Trans. Ph. Am. 5. t. 3. f. 6. 
—U. Dovetasia. 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, 
slightly salmon coloured and irridescent : cardinal teeth double in 
both valves, the lateral straight. 4..1,9,.—China. 
U. Grayanus. (Lea. ~Trans..Am. Ph. 5. t.' 9.) f. 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. +..3,3;.—China. 
U. PonpERosvs. oe Bs D. p. 51.—Mya Crassa. 
Wood. G. C. t. 20. f. 21.—W. 2. f. 28. Ovate, ponderous, 
narrow anteriorly and truncated reer the cartilage slope, the 
beaks decorticated ; cuticle dark brown, longitudinally wrinkled in 
two different directions: primary tooth very strong shutting into 
a corresponding cavity. 33..6.—China. 
* * Nodulous. 
U. Lacurymosus. Lea. on U. 1. p. 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. 14% .. 14.—Ohio. 
N 
