370 



MANUAL OF THE MOLLUSCA. 



area {h) flat ; deltidium (c?) 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 tlie small and deep 

 visceral cavity ; disk grooved for 

 the reception of the looj), the 

 \ grooves separated by branches from 

 a central septum ; looj) often un- 

 symmetrical, lobed, and united more 

 or less intimately with the sides of 

 the grooves; ventral valve (Fig. IGo) 

 deeply excavated, hinge-teeth pro- 

 minent ; cavities for the adductor 

 (a) and pedicle muscles (^j) small ; 

 > disk occupied by two large smooth 

 imjDressions of the cardinal muscles, 

 bordered by a vascular line. 

 Fig. 165. T. radians. -|. 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 ; ejDidermis distinct. 



T. radians is the only unattached species, it is suj)posed to be 

 fixed by a pedicle when young. (D'0rbign5^) 



I 



Dorsal valve. Fig. 166. Profile.* 



fl, adductor ; c, crura ; /, loop ; j, cardinal process ; p, hinge-plate ; s, dorsal septum ; 



V s, ventral septum ; t, dental sockets. 



T. hieroghjpJiicum, PL XY., Fig. 12, has a very complicated 

 * The loop (wliich was discovered by Professor King) has a distinct suture iu Uio 



