442 



MANUAL OF THE MOLLTTSCA. 



is seldom compact, its lamellae are extremely thin, and separated 

 by intervals like the water- chambers of Spondylns ; similar 

 spaces occur in the deposit, filling the umbonal cavity of the 

 long-beaked oysters.* 



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

 the rim of the shell, the cellular structure is often apparent j 



Tig. 231. Section of a fragment of Ostrea Cornucopia. 



•obscure bifurcating impressions radiate from the pallial line to 

 the outer margin (Fig. 232, v, v). 



These have been compared to the vascular impressions of 



Fig. 232. Part of the rim of Hadiolites Mortoni, Mantell.t 



Crania (Figs. 193, 194), and constitute the only argument for 

 suiDposing the Budistes to have been jpaUiohranchiate ; but they 



* The water-chambers in some of the cylindrical Hippurltes are large and regular, 

 like those of the fossil corals Amplcxjis and CyafhophyUuni. A section of HippuriteS 

 l)i-oculatus passing through only one of the dental sockets, resembles an Orthoceraa 

 with a lateral siphunule ; whilst a Caprinella (Fig. 246), which has lost its outer layer, 

 mi^ ht be mistaken for an Ammonite. 



t Traced from the original specimen in the Museum of the School of Mines, b, is 

 the inner edge ; a, the outer edge ; v, v, the dichotomous impressions ; the horizontal 

 laoniuse are seen on the shaded side. Lower chalk ; Sussex. 



