BRACHIOPODA. 



221 



Fig. 127. T. radians. 



THECIDIUM, Defrance. 



Etym. TheUdion, a small pouch. Type, T. radians, PI. XV. fig. 11. 

 Shell small, thick, punctate, attached by the beak ; hinge-area (ft) flat ; 

 deltidium (d) triangular, indistinct : dorsal valve (fig. 127) rounded, de- 

 pressed ; interior with a broad granulated margin ; cardinal process promi- 

 nent, between the dental sockets ; oral processes united, forming a bridge over 

 the small and deep visceral cavity ; disk grooved for the reception of the loop, 

 the grooves separated by branches from a central septum ; loop often unsym- 

 metrical, lobed, and united more or less intimately with the sides of the 

 grooves: ventral valve (fig. 129) deeply 

 excavated , hinge-teeth prominent ; cavities 

 for the adductor (a) and pedicle muscles (p) 

 small ; disk occupied by two large smooth 

 impressions of the cardinal muscles, bor- 

 dered by a vascular line. Animal (fig. 128) 

 with elongated oral arms, folded on them- 

 selves and fringed with long cirri ; mantle 

 extending to the margin of the valves and 

 closely adherent ; epidermis distinct. 



T. radians is the only un-attached spe- 

 cies, it is supposed to be fixed by a pedicle 

 when young (D'Orb.) 



. T. hierofflyphicum, Pi. XV. fig. 12, has a 

 very complicated interior ; whilst in seve- 

 ral others there are but two brachial lobes. 

 The Liassic species form the subject of a monograph by M. Eugene Deslong- 

 champs; they are often minute, and attached in numbers to sea-urchins, 

 corals, and terebratulse. 



Distr. 1 sp. Medit. Fossil, 27 sp. Trias . Europe. 



* Dorsal valve with the animal, magnified. Coll. Davidson. 



Fig. 129. T. radians, y. 



