TRIBE NAYADES. 213 



U. Guarayana. Lea. — Monocondylea 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 a lunule : inside pink, tooth 

 little elevated, almost obsolete in one valve. 1 J . . 1 -J. — S. America. 



U. Fossiculiferus. Lea. — Monocondylea F. JD'Orbig. 

 in Mag. de Z. 1835. Ovate, compressed, thickish, wrinkled by 

 striae, 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. Minuanus. Lea. — Monocondylea M. D' Or big. 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-. . 1J. — S. America. 



U. Margaritiferus.* Dran. 132. t. 10. /. 17, 8, 9.—Pfeif. 

 t. b.f. 11.— Ross. f. 72. 74. 129.— Turt. M. f. 9.— Mya M. 

 Lin.—D. p. 52.— W. t. 2.f. 30.— List. t. \49.—Schrce. F. t. 4. 

 ,/*. 1. — U. Sinuata. Lam. fy Pfeif. — U. Elongata & Roissyi. 

 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 

 in the other. 3. — Europe. 



U. Holstonianus. Lea. — Marg. H. Lea. on U. 2. p. 42. 

 t. 13. f. 37. Transverse, very inequilateral, thinnish, oolong, 

 rounded at both ends and the ventral edge incurved in the middle ; 

 the beaks -somewhat prominent and undulated ; cuticle brown, 

 wrinkled and apparently rayless ; nacre white and irridescent : 

 cardinal teeth large. 1J. . 2^.—R. Holston. 



U. Fabula. Lea.— Marg. F. Lea. on U.2.p. 44. t. 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. 

 \ . . a. — Tennessee. 



