CONCH YLl A—DITHYRA. 



xvu. 



52. DiSCiNA. — Testa discoidea, valvd superior i medio mamillato- 

 productd. 

 Shell disk-shaped, with the upper valve protube- 

 rant in the initklle. 



46. Peoten. — Testa basi aiuicidatd, fovea cardinali trigond. 

 Shell auricled at the base, with a triangular 

 hollow at the hinge 



45. OsTREA. — Testa basi inauriculatd, jfoved cardinali trigond. 

 Shell without auricles at the base, with a trian- 

 gular hollow at the hinge. 



** Cardo dentibus primarUs, lateralibus ntdlis. 



Hinge with primary teeth, but no lateral ones. 



8. Mya. — Testa (equivahis, dente elevato transverso in valvd 

 sinistrd, dextrd mulicd. 

 Shell equivalve, with an elevated transverse tooth 

 in the left valve, and none in the right. 



10. Sphenia. — Testa incequivalvis, dente elevato transverso in 

 valvd sinistrd, dente concavo in valvd dextrd. 

 Shell inequivalve, with an elevated transverse tooth 

 in the left valve, and a concave tooth in the right. 



9 Lyons! A. — Testa incequivalvis, dente mobili comnmni valvas 

 coadunante Joveold utrinquh obliqud. 

 Shell inequivalve, with a conimou moveable tooth 

 uniting the valves together by an oblique cavity 

 in each. 



• ij 



