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, 
iridescent anteriorly. —N. America. 
U. Asper. Lea. on U. 1. p. 95. t. 9. f. 15.—Tr. Am. Ph. 
4. t. 9. f.15. Subquadrate, broader than long, inequilateral, ante- 
riorly angulated, thick, covered with small rough tubercles except 
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. 12..14+.—R. Alabama. 
U. Dorrevitiianus. Lea. on U. 2. p. 73. t. 17. 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 andcurved. 23..2,%.. 
— Ohio. 
U. Srares. 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 umbonesflattened; 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. 13..13.—R. Alabama. 
U. Meranever. Lea.—Con. Mon. U. p.10.t. 5. f. 2.—Raf. 
—U. Nonosus (in. Sil. J. 6. t. f. 7.) and Rucosus. 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}3..2.—N. America. 
U. Cornutus. Barnes. in Sil. J. 6.t.4. f. 5.—U, Reriexvus. 
N 2 
