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 inches f. The form is never expanded, as usual in 

 Ventriculites J 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 {, 

 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 

 poljrpidom which was covered by it. See PI. 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 ha& occurred to me 



* Ante, pp. 41, 203. It was the circumstance of the VentriciilidyD being 

 polypiferous on both surfaces that rendered these provisions so necessary. 

 In Halodactijlus, &c. one surface only is polypiferous. See note J p. 41. 



f 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 tlie 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. 



X 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 ante (vol. xx.) pp. 96, 188. 



