UPPER, OR FLINTY CHALK. 171 



Fig. 9. Eepresents a specimen viewed from beneath. 



The specimens above described exhibit no traces of organization, 

 except at the margin and base, the outer surface of the original being 

 obscured by the silex, in which it is imbedded. In some examples, how- 

 ever, the enclosed zoophyte may be separated from the surrounding flint 

 by a well-directed blow on the margin, and very dehcate casts and 

 impressions may be thus obtained. 



The former exhibit the external integument changed into a white 

 friable carbonate of lime ; the latter form conical cavities covered with 

 numerous interrupted ridges, disposed in a radiated manner. 



The casts of the funnel-shaped cavity are solid cones ; their surface 

 exhibiting numerous minute papillae, that have been moulded in the 

 open extremities of the tubuli. A chalk specimen of this kind is figured 

 by Lhwyd, No. 176* : but it is drawn in an inverted position. 



Tab. xi. The fossils here represented are remarkably interesting, since 

 they tend to elucidate the formation of those above mentioned, and 

 establish the identity of the chalk and flint specimens. 



In fig. 1, a conical flint fills up the lower half of the funnel-Uke cavity, 

 and is surrounded by the impression of the external surface of the upper 

 portion. 



In fig. 2, the stem is occupied by a small turbinated flint, that sends off 

 several radical processes from its base. The dissimilarity in the size and 

 shape of these flints is purely accidental, arising from a greater proportion 

 of silex having been deposited in the one instance, than in the other. Hence 

 we may infer, that if in either example the quantity of silex had been 

 suflicient to have filled the entire cavity, the flints thus formed would, in 

 every particular, have resembled those dehneated in the preceding tablet, 

 figs. 1. 5. 9. 



Among the singular forms assumed by the sihceous specimens of 

 ventriculites, none are apparently more difficult of explanation, than the 



* It is thus described, " Astroitae congener radvlaria cretacea. E puteis cretaceis juxta 

 Aston Rowani in agro Oxoniensi." Lhixyd, Lith.Britt. 



Z 2 



