116 CONCHACEA. 



Family II.— CONCHACEA. 



With at least three primary teeth in one valve, and the same 

 number for the most part in the other; in a few instances less. 



Sub-Division.— FLUVIATILE. 



Shells covered with a spurious epidermis, and the hinge pro- 

 vided with lateral teeth. 



Genus IV.— CYCL AS.— Lamarck. 



Shell thin, equivalve, subequilateral, ventricose, transverse, 

 or semiorbicular ; closed all round ; beaks mostly tumid ; car- 

 dinal teeth very small, sometimes hardly perceptible ; with one 

 plicated or lobed tooth in the left valve, and two divergent in 

 the other ; lateral teeth transversely elongated, acute, com- 

 pressed, or lamelliform, double in the left valve ; clothed with 

 a thin, horny epidermis ; two lateral, ovate, muscular impres- 

 sions ; pallial impression entire, destitute of a sinus ; ligament 

 external. 



1. Cyclas rivicola, pi. XXII, f. 1, 2, 3. 



Cyclas rivicola, Lamarck, V, p. 558; Cyclas cornea, Pfeiffer, 

 Land un Was. Sc, p. 121, pi. 5, f. 3, 4, 5 ; Turton, Brit. Biv., 

 p. 248, pi. 11, f. 13; lb., Man., p. 12, f. 1 ; Fleming, Brit. An., 

 p. 453 ; Alder, Mag. Zool. and Bot., II, p. 118 ; Brown, Illust. 

 Conch., p. 93, pi. 39, f. 16, 17, 18; lb.. First Ed., pi. 17, f. 12; 

 Cyclas cornea, Drapernaud, Hist, des Moll., p. 128, pi. 12, f. 1, 

 2, 3 ; Tellina coy^nea, Wood, pi. 46, f. 3 ; Cardium corneum, 

 Montagu, p. 86. 



Shell thin, pellucid, convex, ovate ; covered by a horny- 

 green epidermis, frequently marked with paler transverse 

 zones; furnished with strong, close, regular, concentric striae; 

 umbones obtuse, central, and slightly turned to one side ; 

 hinge with a very small cardinal tooth in the left valve, which 

 fits into the space between two small, but rather produced 

 teeth in the opposite valve ; both valves provided with pro- 

 minent, lateral, laminated teeth on each side, those of the 

 right valve hardly rising above the margins, but in the left 

 valve projecting considerably beyond them ; inside smooth, 

 bluish-white, dull in (he concave disk, but glossy towards the 



