204 Mr. Toulmin Smith on the Classification 
seems to involve fewer difficulties than, at first sight, strike the 
observer in respect to the other genera. 
The access of sea-water was well secured in this genus, inas- 
much as, though the folds of some species are close, the expan- 
sion of the whole mass and relative size of the central cavity are 
usually greater than in any other genus, thus compensating for 
the frequently looser nature of the fold in the other genera. It 
may perhaps be inferred that the ocean in which this genus 
dwelt was subject to such modifications that its waters were less 
liable to disturbance than those of the ocean in which Cepha- 
lites and Brachiolites originated ; so that a larger size could be 
attained in safety, and with less multiplied provision for secu- 
rity against such disturbances. 
The whole genus Ventriculites is characteristic of the upper 
chalk. I have a few specimens which may possibly have come 
from the middle chalk* ; but every such case is doubtful, while 
I have carefully ascertained that the mass are certainly from the 
upper chalk. 
§ a. Simplices. 
Inner and outer surfaces corresponding. 
I. Ventriculites simplex. PI. VIII. (vol. xx.) fig. If* 
Membrane simple and without trace of fold : moveable processes 
minute or absent. 
This is the type of the whole family, and it is a most happy 
circumstance that it exists. That existence at once destroys all 
the theories as to the anatomy and physiology of this group of 
fossils which have been suggested from examination of some spe- 
cial forms only ; while it affords the means of demonstrating some 
important points. It appears to be a rare species, as, though I 
am fortunate in having myself obtained a very perfect series, I 
have certainly never seen half a dozen specimens out of my owm 
cabinet. 
When the underskin and polyp-skin are absent, as most often 
happens, the surface, if uninjured, is particularly beautiful. It 
exhibits, over its whole surface, with the utmost regularity, that 
square arrangement of fibre, with the subtending octahedra, already 
described. When the under skin is present without the poljq)- 
skin the squares are not seen, but the whole surface is covered 
* See Ann. and Mag. Nat. Hist. vol. xx. p. 337. 
*1* It is not possible to give illustrations, in this work, of all specimens of 
full size, several of them often exceeding in diameter the size of the page. 
It will he understood, therefore, that all the specimens of the present genus 
figured on PI. XIII., with the exception of fig. 9, are of specimens below the 
usual size. 
