210 TESTACEOUS MOLLUSCA. 
lamelle thick and slightly exserted, ligamental sinus large and oval. 
2..4—RKhine, Loire, §c. 
U. Monovontvus. Say. Am. C.t.6 —EKat.in Tr. Jour. Med. 
1831.—U. Sotentrormis. Lea. on U.1. p. 97. Elongated, trans- 
verse, thin, inequilateral, compressed, rounded and sloping from the 
beaks at both ends, ventral margin strongly incurved in the middle; 
the beaks slightly prominent : cuticle very dark brown and wrin- 
kled, the nacre bluish white and irridescent: cardinal tooth a simple 
small tubercle in both valves, the lateral long straight and very im- 
perfect. 131,..43;— Ohio. 
U. Emarernatus. Lea. Trans. Am. Ph. 5. t. 9. f. 22. 
Transversely elongated-oblong, thickish rounded and very short 
posteriorly, anteriorly obtusely subbiangulated, ligamental edge 
straightish, the ventral strongly incurved in the middle and com- 
pressed ; beaks rather elevated and undulated at the tip: ligament 
long and thin, cuticle greenish yellow, becoming green on the ante- 
rior slope, nacre pearly white and irridescent: cardinal teeth small 
oblique and double in both valves, the lateral long and slightly 
curved. 1,3,..24. 
Division B. 
Having one cardinal tooth. 
Section A.—Plicate. 
U. Compianata.—Atasmoponta C. Barnes § Hild.— 
Sympuonota C. Lea. on U. 1. p. 62. Triangular-ovate, inequi- 
lateral, transversely wrinkled, compressed, thick; anterior dorsal 
edge winged connate and obscurely undulated at right angles to the 
wrinkles: the beaks compressed and scarcely projecting, undulated, 
the hgament concealed with an oblique callous plane under it: cuti- 
cle dark brown, nacre white and irridescent : a large cardinal tooth 
in each valve. 3..5.—Ohio, &c. 
U. Conrracosa.—ALasmoponta C. Say. Am. Con. t. 21. 
Transversely suboval, dark olive or blackish, inequilateral, anterior 
edge forming an angle with the scarcely sloping ligamental one ; 
convex, with oblique irregular abbreviated undulations not directed 
towards the beak, and numerous more or less abbreviated, slightly 
raised lines from the umbo becoming obsolete towards the base, 
those in front divaricating towards the hinge margin: beak with a 
groove and a double series of three or four tubercles: inside white 
margined with opaque whitish or violaceous. 33.—-N. America. 
U. Arcura.—Mare. A. Lea. on U. 2. p. 71. t. 22. f. 69. 
Triangular, very inflated, very thin, with about eight transverse 
curved folds, dorsal edge much sinuated and angulated posteriorly : 
yentral edge little rounded and not incurved, the umbones very 

