213 
of the Ventriculidse of the Chalk. 
been based. The inquirer will be well-prepared for this entire 
absence of any such tubuli from its having been seen that^ in the 
preceding species, the depressions (which alone are what could, 
even on a superfieial observation, be called tubuli) have the 
greatest diversity of form and character, a diversity which at 
once negatives their being of the nature assigned to them by 
Dr. Mantell. It has moreover been seen that, as a matter of fact, 
they are in no instance tubuli, but that they can, on the contrary, 
be traced as folds of the membrane forming the polypidom itself. 
On the other hand, the present is the only species which could 
seem to lend any support to the contraetile theory. The observer 
however who has studied the several preceding species will pro- 
bably ask for no proof that the present does not stand out as an 
anomaly and an exception from them all. If he need such, he 
may be referred to the observations already made *, generally, on 
the matter. These have full application in this as in every other 
case, and sufficiently show that this species offers no violation to 
that Unity which prevails through every branch of the present 
inquiry. 
Different specimens vary in the size and depth of the plait. 
It is suggestive, perhaps, as before, of difference of age. It is 
where the plait is deepest and broadest that the moveable pro- 
cesses become most conspicuous. 
The plaits are not narrow at the base and of increasing size 
as they approach the margin. They maintain the same size from 
base to margin, and the increase of surface is effected by the 
increase in number of plaits as the margin is approached ; an in- 
crease effected by the division, from time to time, of one plait 
into two, — each of equal size with the original one, — a mode of 
increase which generally prevails in all species in which the plait 
can be distinctly followed. The species is not common. 
§ b. Complicati. 
Inner and outer surfaces not corresponding. 
I. Ventriculites mammillaris. PI. XIII. figs. 7 & 14. 
Inner plaits simple and regular : outer plaits raised in large hol- 
low bosses at regular intervals : processes very conspicuous : 
thickness of wall considerable. 
This is the first instance in which we find the direction of the 
fold changed between the external and internal surfaces. There 
is no species more strongly marked, and none which affords a 
better illustration of the value and importance of a principle of 
Unity as the essential and most valuable means to true scientific 
* Ante, vol. xx. p. 89. 
