870 MANUAL OF THE MOLLUSCA. 
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 
visceral cayity; 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 
r 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. 
Fig. 165. T.radians, 2, 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.) 
Dorsal valve. Fig. 166. Profile.* 
a, adductor; c,crura; 7, loop; J, cardinal process; p, hinge-plate; s, dorsal septum ; 
v s, ventral septum; ¢, dental sockets. 
T’. hieroglyphicum, Pl. XV., Fig. 12, has a very complicated 
* Tho loop (which was discovered by Professor King) has a distinct suture in the 
