444 



MANTTAL OF THE MOLLtTSCA. 



small, verj' deep, bounded by tbe second duplicature {m) ; tbird 

 duplicature («) projecting into the umbonal cavity : free valve 



Fig. 233. Intei'ior of lower valve, ^, 



Fig. 234. Upper valve (restored). 



Hippurites radiosus, Desm. Lower chalk, St. Mamest, Dordogne.* 

 a, a, adductor impressions and processes ; c, c, cartilage pits ; t, t', teeth and dental 

 sockets; u, umbonal cavity; p, orifices of canals; /, ligamental inflection; tti mus- 

 cular, n siphonal milection. 



depressed, with a central umbo, and two grooves or pits cor- 



7T7 .-^^sfissSS^irT-fiOps^Wi 



Fig. 235. JJ. Toucasianus, upper valve, f .t Fig. 236. Lower valve, with mould, | 

 I ligamental, m muscular, n siphonal inflections ; x, fracture, showing canals } 

 c, cartilage ; u, left umbo ; the arrows indicate the probable dii-cction of the branchial 



responding to tbo posterior ridges in the lower valve ; surface 



* From the original in tlie Brit, Mus. The inner layer of shell in this species has an 

 irregularly cellular structure, to which its preservation is due. 



t This internal mould, representing the fonn of the animal, was obtained by remov- 

 ing the upper valve piecemeal with tlie chisel ; a plaster-cjist taken from it represents 

 the interior of tlie upper valve, with the bases of the teeth and apophyses. See origi- 

 nals in Brit. Mus. 



