TRIBE NAYADES. 187 
U. Cor. Conr. N. F. W. S. p. 28. t.3. f.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 suleated.—R. Alabama. 
U. Truncatus. Smain. Z. Il. 2d series. 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. 12. .33. 
U. Decisus. Lea.on U. 1. 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. 15%,..31.— &. Alabama. 
U. Cravus. Lam.—Conr. Mon. U. p. 5. t. 3. jf. 1.—Mopio- 
LirorMis of Say. (not Lea.)—U. Scatenius. 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}..13.—N. America. 
U. Paruuus. Lea. on U.1. p. 55.(h Tr. Am. 3:) t. 12. f. 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- 
ruptéd rays, nacre pearly white; cardinal tooth small, the lateral long 
and slightly curved. 12..2,3;.— Ohio. 
U. Raveneuianus. 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. °,. . 14.—Tennessee. 
U. Ranaianus. Lea. on U. 2. p. 95. (§ Tr. Am. Ph.)t. 18. 
J. 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 ; 
