TIUBE NAYADES. 217 



retuse. Europe.— Closely resembling Cygnea, but never so dilated 

 posteriorly. 



A. Fragilis. Lam. 4. — Del. t. 13./*. 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. 2^. — Newfoundland. — Ohio. 



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

 lus F. D. p. 316.— List. t. 157. /. 12.— TF. t.' 12. /. 35.— A. 

 Cataracta. Say. in Kick. Enc. Amer. 4. t. 3. f. 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. 2f . . 4%. — Massachusets, fyc. — Very like 

 Cygnea. 



A. Crispata. Lam. 7. — E. t. 203./. 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. 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. 2\. 



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

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

 4.— A. Rugosus. Swain. Zool. II. 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 . . 3J. — Mas- 

 sachusets. 



A. Exotica. L,am. 12. — Del.t. 13./. 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. 5J. —Peru. 



