TRIBE NAYADES. 179 



longer at the anterior side whose margin is generally retuse ; pos- 

 terior side simply rounded, ventral margin incurved before the 

 middle ; yellowish or brownish olive, the entire surface studded 

 with small subequal more or less rounded or transverse protube- 

 rances arranged in lines which in front of the strongly raised 

 umbonal ridge incline towards the ligament and anterior margin, 

 are angulated in the groove and on the middle, and thence curve 

 posteriorly : umbones not very prominent ; within pearly white, 

 irridescent anteriorly. — N. America. 



U. AsPER. Lea. on U. I. p. 95. t. 9./. 15.— T^r. Am. Ph. 

 4. t. 9._/. 15. Subquadrate, broader than long, inequilateral, ante- 

 riorly angulated, thick, covered with small rough tubercles excejit 

 in an oblique furrow from the beak to the ventral margin which is 

 there arcuated ; beaks slightly prominent ; ligament short and thick : 

 cuticle brown and wrinkled, nacre pearly white ; cardinal teeth 

 rather large, lateral slightly curved. If. . 1^. — li. Alabama. 



U. DoRFEuiLLiANUs. Lea. on U. 2. p. 73. t. \7.f. 54. Sub- 

 triangular, inflated, tuberculated, inequilateral, very thick ; beaks 

 large, very prominent, recurved : ligament short and thick : cuticle 

 dark brown and transversely striated, nacre white ; dorsal edge 

 anteriorly very oblique, posteriorly very short and deeply incurved : 

 cardinal teeth large and erect, lateral short and curved. 24 . . 2^"^. 

 — Ohio. 



U. Stapes. Lea. on U. 1. p. 87. t. 7. f. 8. Tr. Am. 4. t. 7. 

 f. 8. Subtriangular, subequilateral, thick, rounded posteriorly ; 

 anterior slope truncated undulated, nearly perpendicular, flat except 

 just behind the short and thick ligament : posteriorly with trian- 

 gular tubercles whose angle is directed ventrally ; umbonal slope 

 tuberculated, the umbones flattened; cuticle yellow, slightly wrinkled, 

 furnished with indistinct small pencil-like markings, nacre pearly 

 white ; cardinal teeth rather large, lateral distinct from the cardinal 

 and pointing to the ventral margin. Ig . . If. — i?. Alabama. 



U. Metanever. Lea. — Con. Man. U. p. 10. t. o.f. 2.—Baf. 

 — U. NoDosus (?"?i. Sil. J. 6. t. f. 7.) and Rugosus. Barnes. 

 Subquadrate, thick, ventricose, posteriorly rather shorter and 

 rounded above and below ; studded except near the edge with 

 large irregularly disposed tear-like tubercles ; anteriorly angulated 

 above and below, the incurved edge obtusely forming a rectangle 

 with the almost direct ligamental edge, ventral edge scarcely arcu- 

 ated ; disc with an oblique radiating furrow ; umbonal slope very 

 prominent, broad, rounded, and rough with numerous tubercles 

 (some very large) : anterior slope wide, concave or angular, 

 with oblique slightly recurved series of rib-like tubercles : umbo 

 slightly prominent, beaks retuse : cuticle yellowish brown with 

 interrupted green rays ; within white. 1| . . 2. — N. America. 

 U. Cornutus. Barnes, in Sil. J. G. t. 4.f. 5. — U. Reflexus. 



N 2 



