CONCHIFERA. 



281 



Fig. 192. Section of a fragment of Ostrea cornucopia. 



The inner layer ceases at the pallial line, beyond which, on the rim of the 

 hell, the cellular structure is often apparent; obscure bifurcating impres- 

 ions radiate from the pallial line to the outer margin, (fig. 193, v, v.} 



Fig. 123, Part of the rim of Radiolites Mortoni, Mantell, * 

 These have been compared to the vascular impressions of Crania, (figs. 

 57, 8) and constitute the only argument for supposing the Eudistes to have 

 >een palliobranchiate ; but they occur on the rim of the shell, and not on the 

 , as in Crania.^ The chief peculiarity of the Hippuritidte is the dissimi- 

 arity in the structure of the valves, but even this is deprived of much signi- 

 icance by its inconstancy 4 The free valve of Hippurites is perforated by 

 adiating canals which open round its inner margin, and communicate with 



