Cyclas. MOLLUSCA. CYCLADjE. 453 



behind, with prominent edges for the cartilage, which is dis- 

 tinct. 



Pectunculus subviridis, List. Conch, t. clix. f. 14 — Cyclas cornea, Drap. 

 Moll. 128. t. x. f. 1, 2, 3 — Cyclas rivicola, Lam. Hist. v. 558.— Turt. 

 Biv. Brit. 248, t. xi. £'13 Slow running streams, England. 



Length T 6 5 ths, breadth T 7 5 thsof an inch ; cuticle yellowish or olive; often 

 darker, with pale bands ; less tumid, and more regularly curved in the out- 

 line than the preceding, of which it was supposed to be only a large variety 

 by Lister who had it from Doncaster, Linnaeus from Iceland, and Montagu 

 from the Thames. The transverse grooves and coloured bands of both spe- 

 cies, depending on circumstances accelerating or retarding the growth of the 

 animal, furnish characters of uncertain value, though generally employed by 

 modern authors. 



54<6. C. lacustris. — Subinequilateral, with minute concentric 

 strise ; beaks prominent, with the margin in front thin and ele- 

 vated. 



Tellina lacustris, Mull. Hist. Verm. ii. 204. — Cardium lac. Mont. Test. 



Brit. 89 Cyclas lac. Drap. Moll. 130, t. x. f. 6. 7 — Turt. Biv. Brit. 



249, t. xi. f. 18. — In slow running streams, England. 



Length T 3 5 ths, breadth T 4 5 ths of an inch ; thin, glossy, transversely sub- 

 rhomboidal ; margin thin. 



~- 547. C. amnicus. — Inequilateral, transversely ovate, and sul- 



cated concentrically. 



Tellina amnica, Mull. Hist. Verm. ii. 205. — T. rivalis, Maton, Linn. 

 Trans, iii. 44, t. xiii. f. 37, 38. — Cardium amnicum, Mont. Test. Brit. 



86 Cyclas palustris, Drap. Moll. 131, t. x. f. 17, 18 — C. amnica, 



Turt. Biv. Brit. 250, t. xi. f. 15. — In slow running streams. 



Length about three-eighths, breadth half an inch ; greenish ; slightly pro- 

 duced anteally ; the margin in front of the beaks prominent ; the furrows of 

 the sulci are finely striated. 



EXTINCT SPECIES. 



1. C. deperditus Ovato-transverse, rather gibbous, umbonate ; lines of 



growth elevated, irregular ; central hinge teeth three, lateral ones two 



Sower. Min. Conch, t. clxii. f. I. —Plastic Clay. 



2. C. cuneiformis — Transversely cuneate-ovate, gibbous ; lines of growth 

 numerous, fine ; central hinge teeth three ; lateral ones two. — C. deperdita, 

 Park. Org. Rem. 111. 189, t. xiii. f. 5. — C. cun. Sower. Min. Conch, t. clxii. 

 f. 2, 3. — Plastic Clay. 



3. C obovatus — Obovate, gibbous, anterior side obtuse ; beaks large ; cen- 

 tral hinge teeth three ; lateral ones two. — Sower. Min. Conch, t. clxii. f. 4, 5, 

 6. — Plastic Clay. 



4. C. medius. — Transversely obovate, depressed, thick, smooth, anterior (?) 

 side small, posterior rather pointed ; one tooth near the beaks in each valve. 

 — Cyrena media, Fitton, Annals of Phil. Nov. 1824, 376. Cyclas medius, 

 Sower. Min. Conch, t. Dxxvii. f. 2.— In the fresh water Formation between the 

 Green and Iron Sand. 



5. C. membranaceus — Transversely obovate, depressed, smooth, very thin ; 

 anterior side small, posterior rather pointed. — Cyrena memb. Filton, Annals 

 of Phil. Nov. 1824, 376. Cyclas memb. Sower. Min. Conch, t. uxxvii. f. 3. 

 — Weald Clay under Green Sa nd. 



