210 Mr. Toulmin Smith on the Classification 
I can as clearly see the fold in and out of that membrane as it is 
seen in fig. 11. PI. 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 6 of Dr. MantelFs ^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 speeies 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 beeome 
converted into chalcedony. The outer surface is nearly entire, as 
is also a eonsiderable part of the inside. The structure is, as 
usual in solidified ehaleedonic specimens obliterated throughout 
the greater part, but it can, in places, be still clearly traeed. The 
individual underwent pressure before or during the proeess of 
fossilization, and also some decomposition, so that the external 
charaeter 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 ehaleedonic without 
the presence of any true flint, — renders it however very in- 
teresting J. 
4. Ventriculites muricatus. PI. XIII. figs. I & 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 Ventriculides preserved in flint, and leaves an exquisitely delicate 
cast of the body of the animal and of the structure of its polypidom. 
T See before, p. 85. vol. xx. 
J It has already been stated (vol. xx. p. 86), that “ in general when any 
part of the soft substance of a body encased in chalk decomposed, its place 
was soon filled 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. 
