FOSSILS OF THE WHITE CHALK. 
295 
Fig. 251. Fig. 252. 
Osfrea vesicularis. Syn. Gryphcea Inoceramus LamarcMi. Sjn. Ca- 
convexa. Upper chalk aud Upper tillus Laniarckii. White chalk 
greensand. (Dixon’s Geol. Sussex, Tab. 28, 
Fig. 29). 
of southern Europe, a single representative only (Fig. 253) 
has been discovered in the white chalk of England. 
Fig. 253. Fig. 254. 
Radiolites Mortoni, Mantell. Houghton, Sussex. White chalk. Diameter 
one-seventh natural size. 
Fig. 253. Two individuals deprived of their upper valves, adhering together.—Fig. 
254. Same seen from abnve\—Fig. 255. Transverse section of part of the wall of 
the shell, magnified to show the structure.—Fig. 256. Vertical section of the same. 
On the side where the shell is thinnest, there is one external furrow’’ and corre¬ 
sponding internal ridge, «, 5, Figs. 253, 254; but they are usually less prominent 
than in these figures. The upper or opercular valve is w’anting. 
The general absence, of univalve mollusca in the white 
chalk is very marked. Of bryozoa there is an abundance, 
such Eschara and Escharina (Figs, 257 , 258 ). These and 
other organic bodies, especially sponges, such as Ventriculites 
