442 



MANtTAl, OF THE MOLttlSCA. 



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

 by intervals like the water- chambers of Spondylus ; similar 

 spaces occur in the deposit, filling the nmbonal cayity 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 ; 



Fig, 231. Section of a fragment of Ostrea Carniccopice, 



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 Radiolites Mortoni, Mantell.f 



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

 supposing the Budistea to have been palliobranchiate ; but they 



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

 like those of the fossil corals Amplexus and Cyathophyllum. A section of Hippurites 

 bi'oculatus passing tlirough only one of the dental sockets, resembles an Orthoceras 

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

 mi;.htbe mistaken for an Ammonite. 



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

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

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



