442 MANUAL OF THE MOLLUSCA. 
is seldom compact, its lamelle are extremely thin, and separatod 
by intervals like the water-chambers of Spondylus; similar 
spaces occur in the deposit, filling the umbonai cayity of the 
long-beaked oysiers.* 
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 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 
CI 180s —~y\ 68! 
eH HAS {y ice 
LOTTI A 
a VO 
a as 
Fig. 232. Part of the rim of Radiolites Mortoni, Mantell.t 
Crania (Figs. 193, 194), and constitute the only argument for 
supposing the Rudistes 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 Amplerus and Cyathophyllum. A section of Hippurites 
bi-oculatus passing through only one of the dental sockets. resembles an Orthoceras 
with a lateral siphunvle; whilst a Caprinella (Fig. 246), which has lost its outer layer, 
mi: ht be mistaken for an Ammonite. 
+ Traced from the original specimen in the Museum of the School of Mines. 6, ia 
the inner edge; a, the outer edge; v, v, the dichotomons impressions; the horizontal 
laminge are seen on the shaded side, Lower chalk; Sussex. 
