CONCHIFERA. 



285 



Chama, the mantle-opening for that organ would have been completely ob- 

 structed by the adductor, but that the muscular support was hook-shaped 

 (fig. 200, a). The posterior adductor-process is similarly under-cut for the 

 passage of the rectum, which in all bivalves emerges between the hinge and 

 posterior adductor, winds round outside that muscle and terminates in the 

 line of the exhaleut current. There is a groove (sometimes an inch deep) 

 round the second and third duplicatures in the upper valve, which seems in- 

 tended to facilitate the passage of the alimentary canal, and the flow of water 

 from the gills into the exhaleut channel. The smallness of the space for the 

 branchiffi may have been compensated by deep plication of those organs, as 

 in Chama and Tridacna, 



Fossil, 16 sp. Chalk. Bohemia, Tyrol, France, Spain, Turkey, Syria 

 Algeria, Egypt. 



Fig. 202. Interior of lower valve. Fig. 203. Interior of upper valve. 



Radiolitesmammillaris, Math. | L. Chalk. S. Mamest, Dordogne. 



^, ligamental inflection ; m, pallial line ; c, c, cartilage pits ; a, a, adductor impressions 



and processes ; t, teeth and dental sockets. 



Etym. Radius, 



Radiolites, Lamarck, 1801. 

 ray. Syn. Sphserulites, De la Metherie, ] 805. 



Fig. 204. Side views of the upper valve of iJ. mammillar'ts-^ I, ligamental inflection 

 t, teeth ; a, a', muscular processes. 



S/iell inversely conical, bi-conic, or cylindrical ; valves dissimilar in 



