54 INTRODUCTION. 
Gex. 6. Donax.—Animaltethys;testa 
bivalvis, margine antico obtusissi- 
mo ; cardo, dentibus duobus , mar- 
ginalique solitario , subremoto sub 
ano. 
GEx. 7. Venus.—Animal tethys; testa 
bivalvis, labiis margine antico in- 
cumbentibus ; cardo , dentibus tri- 
bus, omnibus approximatis, latera- 
libus apice divergentibus; vulva et 
anus distincta. 
Gex. 8. Spondylus.—Animal tethys; 
testa inæquivalvis rigida; cardo , 
dentibus recurvis cum foraminulo 
intermedio. 
GEN. 9. Chama.—Animal tethys ; testa 
bivalvis grossior; cardo, callo gib- 
bo,oblique inserto fossulæ obliquæ, 
vulva clausa absque nymphis. 
GEN. 10. Arca.—Animal tethys; testa 
bivalvis, æquivalvis ; cardo denti- 
bus numerosis , acutis, alternis, in- 
gertis. 
