TRIBE’ NAYADES. 217 
retuse. Hurope.— Closely resembling Cygnea, but never so dilated 
posteriorly. 
A. Fraeriis. Lam. 4.— Del. t. 13. f.2. Narrow ovate, thin, 
very fragile, rhombic and compressed, anteriorly short and pos- 
teriorly rounded, with distant transverse grooves, ventral edge entire : 
beaks slightly prominent and undulatingly wrinkled; cuticle, 
greenish olive : inside bluish. 24.—Nemfoundland.— Ohio. 
A. Fiuviatitis. Lea.— Gould. Mas. p. 117. f. 80.—Myti- 
Lus F. D. p. 316.—List. t. 157. f. 12.—W. t. 12. fi 35.—A. 
Cararacta. Say. in Nich. Enc. Amer. 4. t. 3. f. 4.—Lam. 
5.—A. Mareinatus. Say.—A. Terres. Conrad. Suboval, 
thin, inflated ; hinge margin straight, posteriorly imperfectly angu- 
lated and nearly as high as it is anteriorly where it forms an obtuse 
angle at the termination of the ligament and declining in a straight 
line forms a somewhat produced blunted point (this margin is com- 
pressed into a sort of crest); ventral edge much arcuated ; beaks 
moderately elevated, situated at the posterior two-fifths of the shell, 
minutely undulated at the tip : cuticle deep grass green, obscurely 
rayed, darker above the anterior ridge ; nacre silvery or tinged with 
bluish or yellowish. 23..43}.—Massachusets, §'c.—Very like 
Cygnea. 
A. Crispata. Lam. 7.—£.t. 203. f.3. Oblong-ovate, rather 
depressed, thin, yellowish brown, contracted in the middle, with 
small crowded radiating flattened ribs decussated by numerous 
arched and undulated furrows. 2.—Beaks situated at two-sevenths 
from the narrowed and rounded posterior extremity. 
A. Untorsis. Lam. 8. Oblong-ovate, anteriorly subangulated, 
brown, transversely striated; beaks rather prominent with the 
ligament passing between them and the hinge : within bluish to- 
wards the margin and the hinge plate thickish and terminated 
posteriorly by a somewhat prominent callus. 24. 
A. Unpunata. Say. in Nic. Enc. Am. 4. t. 3. f. 6.—Gould. 
Mass. p. 120. f. 79.—A. Pensytvanica. Lam. 9.—Del. t. 13. f. 
4.—A. Rucosus. Swain. Zool. Il. t. 96. Oblong-ovate, thick, 
posteriorly rather shorter and rounded, anteriorly very obtusely bi- 
angulated, dorsal and ventral edges subparallel and but little 
arcuated: beaks prominent, with four or five small undulations ; 
cuticle dark brown, radiated, coarsely wrinkled towards the ventral 
edge; inner dise pale salmon, inner margin broad, olive; hinge 
margin undulated and with the vestige of a tooth. 2..33.—Mas- 
sachusets. 
A. Exorica. Lam. 12.— Del.t.13.f.1. Ovate-oblong, thick, 
transversely grooved and the cuticle olive green, much attenuated 
posteriorly ; posterior end of hinge margin terminated by an angle ; 
ventral edge posteriorly incurved ; beaks prominent, cardinal sinus 
large ; irridescent within. 53.—Peru. 
