= 
446 MANUAL OF THE MOLLUSCA. 
of the yalyes 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 Chama, the mantle-opening for that organ 
would have been completeiy 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 
\ 
NA 
MQ 
WY AN Ny } ), /) 
NY y N 
CY Nice ‘i Hn | Yj 
\\ Ap 
WN \ \ ll 1 i wy), 
Fig. 239. Aippurites cornu-vaccinum. Fig. 240. Radiolites cylindraceus, é. 
Longitudinal sections taken through the teeth (¢, t') and apophyses (a, a’). 
d outer, 7 inner shell-layer ; J, dental plate of lower valve; uv. umbonal cavity of upoee 
valves ; 2, intestinal channel. Originals in Brit. Mus. 
and posterior adductor, winds round outside that muscis, 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 spacs for 
the branchize may have been compensated by deep plication of 
those organs, as in Chama and Tridacna. 
Fossil, 30 species. Chalk. Bohemia, Tyrol, Frauce, Spain, 
Turkey, Syria, Algeria, Egypt. 
RADIOLITES, Lamarck, 1801. 
Etymology, radwus, a ray. 
Synonym, Spherulites, 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, dividing 
