TUIBE NAYADES. 21? 



retuse. Etiroj)e. — Closely resembling Cygnea, but never so dilated 

 posteriorly. 



A. Fragilis. L(im. a. — Del. t. 13._/". 2. Narrow ovate, tliin, 

 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 blnish. 2^. — Nenifoundland. — Ohio. 



A. Fluviatilis. Lea. — Gould. Mas. p. 117./. 80.— Myti- 

 Lus F. D. p. 316.— List. t. 157. /• ]2.— W. t. 12. f. 35.— A. 

 Cataracta. Say. hi Nich. Eiw. Amer. 4. t. 3. _/'. 4. — Lam. 

 5. — A. Marginatus. Say. — A. Teres. 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. 2J . . 4^. — Massachusets, c^c. — Very like 

 Cygnea. 



A. Crispata. Lam. 7' — E.t.203.J'. 3. Oblong-ovate, rather 

 depressed, thin, yellowish bi'own, contracted in the middle, with 

 small crowded radiating flattened ribs decussated by numerous 

 arched and undulated furrows. 2. — Beaks sitiiatedat two-sevenths 

 from the narrowed and rounded posterior extremity. 



A. Uniopsis. 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. 2j. 



A. Undulata. Say. in Nic. Enc. Am. 4 t. 3.f. 6. — Gould. 

 Mass.p. 120./. 79.— A. Pensylvanica. Lam. 9.— Del. t. 13./ 

 4. — A. RuGosus. Swain. Zool. LI. 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 disc pale salmon, inner margin broad, olive ; hinge 

 margin undulated and with the vestige of a tooth. 2 . . 3\. — Mas- 

 sachusets. 



A. Exotica. Ljam. 12. — Del.t. \3.J'. ]. 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; irridesicent within. 5^. —Peru. 



