CONCHIFERA. 



281 



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

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

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

 sions radiate from the pallial line to the outer margin, (fig. 193, v, v.) 



Fig. 123, Part of the rim oi Radiolitcs Mortoni, Mantell, * 

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

 157, 8) and constitute the only argument for supposing the liudistes to have 

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

 disk, as in Crania.\ The chief peculiarity of the EifjiuritidcB is the dissimi- 

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

 ficance by its inconstancy.:|; The free valve of Hippurites is perforated by 

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



« Traced from the original specimen in the Museum of the School of Mines, h, 

 is the inner edge: a, the outer edge; v,v, the dichotoraous impressions ; the hori- 

 zontal laminae are seen on the shaded side. Lower Chalk ; Sussex. 



t M. D'Orbigny considers they were produced by peculiar appendages to the 

 mantle-margin, Avhich, in Hippurites, were prolonged into the canals of the upper 

 valve. 



I The lower valves of some Spondyli are squamous or spiny, the upper plain ; 

 those of many oysters Pectens and some Tellens are diversely sculptured; but in no 

 instance is the internal structure of the two valves different ? The inconstancy of 

 the shell-structure in the Rudista has a parallel in Rhynchonella and Terehratula 

 (p. 213), and in the condition of the hepatic organ in Tritonia and Dendnmohis. 



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