280 Mr. Toulmin Smith on the Classification 
The provisions found through the whole family of Ventricu- 
lidse for ensuring the free access of sea- water to all parts of the 
surface, and for securing permanence of form as one great means 
to that end, have been already noticed* * * § . The present genus 
offers fresh and most remarkable illustrations of those provi- 
sions. 
In every species of this genus the fold is, comparatively to the 
size of the whole body, much deeper and broader — in many 
species positively much deeper and broader — than in any species 
of the genus Ventriculites. The size also is much smaller than 
the average size of the Ventriculites ; the height of specimens 
of the present genus seldom exceeding two inches, rarely attain- 
ing three inchest- The form is never expanded, as usual in 
Ventriculites, but, with few exceptions, approaches nearly to the 
cylindrical, as in V. tenuiplicatus. 
Extent of surface was thus gained in this genus by the increased 
depth and complexity of the fold. But this depth and complexity 
would endanger the safety of the polypiferous surface were there 
no special provision for maintaining the normal position of the 
individual plaits. This was perfectly effected, and at the same 
time with great simplicity and beauty, by stretching across the 
flat upper edges, or, in a few cases, the more prominent points J, 
of the plaits a simple and entire membrane §, which, spread over 
the whole breadth of those edges and from point to point of those 
prominences, retained all the plaits securely in their position ; 
thus ensuring the safety of the whole colony and of the entire 
polypidom which was covered by it. See PL XIV. 
The general constancy in the size and form of specimens of 
this genus throws difficulties in the way of the question of growth. 
It is not easy to understand why we do not find young indivi- 
duals of this genus as of Ventriculites. It has occurred to me 
* Ante, pp. 41, 203. It was the circumstance of the Ventriculida? beiug 
polypiferous on both surfaces that rendered these provisions so necessary. 
In Halodactyhis, &c. one surface only is polypiferous. See note J p. 41 . 
I Hence all the figures of this genus are of specimens of average size. I 
have much pleasure in acknowledging here the pains and care bestowed by 
Mr. Sowerby over these plates. The novelty of the forms and structure 
presented many difficulties, especially as the engravings were made only 
from my drawings. But nothing can be more generally successful or truth- 
ful than the figures which Mr. Sowerby has realised. 
J These latter cases form, however, no exception to the principle of the 
marginal edge of the plaits being always attached to the cephalic membrane. 
The cases in which prominent points of the plaits are attached to the head 
are cases of an additional provision for security. In those cases, as in all 
others, the marginal edge of the membrane, after having undergone all its 
varied modifications of fold, reaches and is attached to the head. See the 
description of C. campanulatus and C. constrict us. 
§ As to structure and nature of this see aiite (vol. xx.) pp. 96, 188. 
