TRIBE NAYADES. 213 
U. Guarayana. Lea.—Monoconpytea G. D’Orbig. S. 
Am. t. 68.f. 4, 5,6, 7. Ovate, inflated very thick, striated, pos- 
teriorly short and rounded, anteriorly marked with two deep sulci, 
a prominent upper keel, and a large groove in the middle of the 
shell: beaks little prominent ; with alunule: inside pink, tooth 
little elevated, almost obsolete in one valve. 14..13.—S. America. 
U. Fossicutirerus. Lea.—Monoconpyura FEF. DOrbig. 
in Mag. de Z. 1835. Ovate, compressed, thickish, wrinkled by 
striz, greenish brown, posteriorly much shorter and rounded, an- 
teriorly very broad and extremely dilated, seeming almost winged, 
the ligamental edge being above the level of the beaks and angulated 
at its extremity, umbonal ridge sharp, ventral edge much arcuated, 
beaks obtuse : within irridescent, white with violet spots, a tooth 
and fossicle in each valve. 3.—S. America. 
U. Minvanus. JZea.—Monoconpyitea M. D’Orbig. S. 
Am. t. 70. f. 8,9, 10. Ovate-oblong, thick, inflated, smoothish or 
wrinkled, posteriorly short, rounded, and the dorsal edge incurved, 
anterior dorsal edge straight and angulated at its extremity : beaks 
prominent and lunate ; cuticle greenish, marked anteriorly with one 
or two greenish lines: inside greenish white, the tooth lamellar 
elevated obtuse. 14..13.—S. America. 
U. Marcaritirervs.* Dr ap. 132. t. 10. f. 17, 8, 9.—Pfeif. 
t. 5. Ff. Vic Ross. f. (72. 74, .129..—-Turt. M. f. 9.—Mya M. 
Lin.—D. p. 52.—W. t. 2. f. 30.— List. t. 149.— Schree. F. t. 4. 
J. 1.—U. Stnvara. Lam. § Pfeif.—U. Evoneata & Rotssyt. 
Michel. Elongated-oblong, transverse, inequilateral, rather com- 
pressed, thick, heavy, black brown, umbones usually eroded, and 
the ventral edge (save in the young,) incurved centrally and little 
arcuated; anterior side twice as long as posterior; its dorsal edge 
short with an obtuse angle at its end rounded posteriorly ; a short 
crested irregular posterior tooth in one valve, shutting between two 
inthe other. 3.—Huwrope. 
U. Hotsronranus. Lea.—Mare. H. Lea. on U. 2. p. 42. 
t. 13. f. 37. Transverse, very inequilateral, thinnish, oblong, 
rounded at both ends and the ventral edge incurved in the middle ; 
the beaks somewhat prominent and undulated; cuticle brown, 
wrinkled and apparently rayless 3 ; nacre white ‘and irridescent : 
cardinal teeth large. 14..22.—R. Holston. 
U. Fasura. Lea—Marc. F. Lea. on U. 2. p. 44.¢. 13. f. 
39. Transverse, inequilateral, somewhat inflated, thickish, flattened 
on the side, suboblong, ventral edge incurved, umbonal slope raised ; 
beaks rather prominent, ligament short ; cuticle greenish with in- 
distinct rays, nacre salmon coloured : cardinal teeth small and erect. 
3 ..4.— Tennessee. 
