100 CONCHIFERA. DIMYAIRA 



Gen. 24. CYRENA, Lam. Venus, Gmel. 

 Shell roundish, trigonal, ventricose, solid, with the beaks decor- 

 ticated ; hinge with three teeth in each valve ; lateral teeth 

 two, one of which is near the primary ones ; ligament exterior. 



The shells of this genus are all exotic, and inhabit large rivers. Some have been 

 found fossil in Europe and America. 



* Lateral teeth crenulated or serrated. 



C. orientalis, Lam. Shell trigonal, olivaceous, with transverse re- 

 mote furrows ; lateral teeth serrated ; umbones violet. f inch 

 long. Inhabits rivers in China and the Levant. Lam. v. 552. 



C.ftuminen, Lam. Shell cordiform, gibbous ; greenish yellow, with 

 concentric furrows ; white within, tinged with violet. 1 inch long. 

 Inhabits Chinese rivers Chem. pi. 30, fig. 322, 323. 



** Lateral teeth entire. 



C. Caroliniensis, Bosc. Shell heart-shaped, turgid, inequilateral; 

 umbones distant, carious, decorticated. 2 inches long. Inhabits 

 rivers in north America. Bosc. Hist. Nat. Coq. iii. pi. 18, fig. 4. 



C. Zeylanica, Lam. Shell somewhat heart-shaped, tumid, inequi- 

 lateral, anterior side subangulated ; epidermis greenish, and the 

 striae fine and unequal. Nearly 3 inches long. Inhabits rivers 

 in Ceylon. Ckem. vi. pi. 32, fig. 336. 



Gen. 25. CYCLAS, Lam. Tellina, Lin. 

 Shell oval, gibbous, transverse, equivalve, with protuberant beaks ; 

 primary teeth very small, sometimes almost none ; or two in 

 each valve, of which one is bent in two ; or only one plicated 

 or lobed tooth in one valve and two in the other; lateral teeth 

 elongated transversely, compressed, lamelliform; ligament ex- 

 terior. 



The shells of this genus are small, of a grayish green or yellowish colour, smooth 

 or striated transversely, and sometimes with slightly coloured bands. 



C. rivicola, Leach. Shell subglobose, equilateral, finely striated 

 concentrically, greenish horn-colour, with paler bands ; bluish 

 within ; a distinct oval impression in front of the beaks, and ano- 

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

 tinct, f inch long. Inhabits rivers in Europe. B. Lister, 

 pi. 159, fig. 14. 



C. cornea, Lam. ( Tellina, Lin.) Shell subglobose, slender, fine- 

 ly striated concentrically, horn-coloured, often with a pale band ; 

 no impression in front of the beaks ; ligament indistinct exter- 

 nally. 4 lines long. Inhabits rivers of Europe. B Brown's 

 Illust. pi. 17, fig. 12. 



C. lacustris, Drap. Shell subinequilateral, with minute concentric 

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

 vated. 3 lines long, 4 broad. Inhabits slow running streams in 

 England. B. Browns Illust. pi. 17, fig- 13. 

 Fossil species of this genus are found in Britain in plastic day, and in the fresh 



water formation, between the green and iron-sand Fleming, 453. 



