446 



MAKFAL OB THE MOILTJSCA. 



of the valves produces central umbones, and necessitates an 

 internal cartilage ; this again causes the removal of the teeth 

 and adductors farther from the hinge-margin, to a position in 

 "which the muscles must have been unusually long, unless sup- 

 ported in the manner described. Supposing the animal to have 

 had a small foot, like ChawM, the mantle- opening for that organ 

 would have been completely obstructed by the adductor, but 

 that the muscular support was hook-shaped (Fig. 239, a). The 

 posterior adductor-process is similarly under-cut for the passage 

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



Fig. 239. Hippurites comu^vaccinum. Fig. 240. Hadiolites cylindraceus, f. 

 Longitudinal sections taken through the teeth (f, V) and apophyses (a, a'). 



d outer, r inner shell-layer ; I, dental plate of lower valve ; u, umbonal cavity of upper 

 valves ; i, intestinal channel. Originals in Brit, Mus. 



and posterior adductor, winds round outside that muscle, and 

 terminates in the line'of the exhalent current. There is a groove 

 (sometimes an inch deep) round the second and third duplica- 

 tures in the upper valve, which seems intended to facilitate the 

 passage of the alimentary canal, and the flow of water from the 

 gills into the exhalent channel. The smallness of the space for 

 the branchiae may have been compensated by deep plication of 

 those organs, as in Chama and Tridacna. 



Fossil, 30 species. Chalk. Bohemia, Tyrol, Prance, Spain, 

 Turkey, Syria, Algeria, Egypt. 



Eadiolites, Lamarck, 1801. 



Etymology, radius, a ray. 



Synonym, Sphserulites, De la Metherie, 1805. 



Shell inversely conical, bi-conic, or cylindrical ; valves dis- 

 similar in structure ; internal margins smooth or finely striated, 

 .simple, continuous ; ligamental inflection very narrow, dividins' 



