370 



MAKtrAL 01? THE MOLLtTSCA. 



area {h) flat ; deltidium {d) triangular, indistinct ; dorsal valve 

 (Fig. 163) rounded, depressed; interior with a broad granulated 

 margin; cardinal process prominent, between the dental sockets ; 



oral processes united, forming a 

 bridge over the small and deep 

 yisceral cavity ; disk grooved for 

 the reception of the loop, the 

 grooves separated by branches from 

 a central septum ; loop often un- 

 symmetrical, lobed, and united more 

 or less intimately with the sides of 

 the grooves ; ventral valve (Fig. 165) 

 deeply excavated, hinge-teeth pro- 

 minent ; cavities for the adductor 

 (a) and pedicle muscles (p) small ; 

 disk occupied by two large smooth 

 impressions of the cardinal muscles, 

 bordered by a vascular line. 



Animal (Fig. 164) with elongated 

 oral arms, folded on themselves and fringed with long cirri ; 

 mantle extending to the margin of the valves and closely ad- 

 herent ; epidermis distinct. 



T. radians is the only unattached species, it is supposed to be 

 fixed by a pedicle when young. (D'Orbigny.) 



Fig. 165. 



T. radians. -^ . 



Dorsal valve. Fig. 166. Profile.* 



a, adductor ; c, crura ; I, loop ; J^ cardinal process ; p^ hinge-plate ; s, dorsal septum ; 



V s, ventral septum ; t, dental sockets. 



T. hieroglypMcum^ PI. XV., Fig. 12, has a very complicated 

 * The loop (wliich WBB discovered by Professor King) has a distinct suture in Iho 



