61 



I can as clearly see the fold in and out of that membrane as it is 

 seen in fig. 1 1 . PL XIII. I wish to be specific and precise on this 

 point because I know that a bare description will be questioned. 

 1 am obliged to state that such a reality as is represented " mag- 

 nified" on p. 279. fig. 2 b of Dr. Mantell's ' Medals of Creation' 

 never had existence * in the nature of the animal of which fig. 2 

 gives a view of the natural size of the fossilized remains. 



In the museum of the Yorkshire Philosophical Institution 

 there is a fragment of a specimen of this species almost discoidal, 

 which still measures more than nine inches across, and, when 

 entire, must have been of still greater size. This is an unusually 

 large size for the species to attain, but the variation in that re- 

 spect is very great. There is another interesting specimen of 

 this species in that museum in which the whole body has become 

 converted into chalcedony. The outer surface is nearly entire, as 

 is also a considerable part of the inside. The structure is, as 

 usual in solidified chalcedonic specimens f, obliterated throughout 

 the greater part, but it can, in places, be still clearly traced. The 

 individual underwent pressure before or during the process of 

 fossilization, and also some decomposition, so that the external 

 character has become somewhat modified, and it is only in parts 

 that the internal arrangement can, at places of section, be seen. 

 The peculiar state of the specimen, solidly chalcedonic without 

 the presence of any true flint, renders it however very in- 

 teresting J. 



4. Ventriculites muricatus. PL XIII. figs. 1 & 12. 

 Membrane deeply folded, without regular figure : folds unequal 

 in width throughout their depth: moveable processes often 

 conspicuous : wall of moderate thickness. 



The point wherein this species differs most essentially from 

 the last will be at once understood by comparing figs. II & 12 

 of PI. XIII. It has been already explained that, through the uni- 

 formity in the breadth of the fold, the general surface of V. quin- 



* And see p. 280 : the polyparium of this species is no more " calca- 

 reous " than that of any other of the Ventriculidae. Treated with acid it is 

 acted on in precisely the same way and to the same extent as are all speci- 

 mens of Venlriculid'ae preserved in flint, and leaves an exquisitely delicate 

 cast of the body of the animal and of the structure of its polypidom. 



f See before, p. 13. 



j It has already been stated (p. 14), that " in general when any part 

 of the soft substance of a body encased in chalk decomposed, its place was 

 soon rilled up with particles of chalk." The above instance is one of the 

 rare exceptions to this general fact; and the peculiar condition of the 

 Yorkshire chalk (which is much more compact than that of Kent, &c.) may 

 lead us to expect more numerous instances of these exceptions from that 

 region than from the chalk of the south-east of England. 



