72 



Genus CEPHALITES. 



Character. Pouch-shaped : very constant in size and dilatation : 

 cavity usually regular and with a single opening ; sometimes 

 winding and with more openings than one : membrane form- 

 ing the wall of the cavity always deeply folded : marginal 

 edges and, sometimes, most prominent points of the plaits 

 attached to a simple apolypiferous membrane stretched across 

 their whole breadth and forming the upper margin or head of 

 the wall : membrane of wall polypiferous on both external and 

 internal surfaces. 



The differences between the genera Cephalites and Ventriculites 

 are so broadly marked that, except in one or two species, it would 

 be difficult to confound even fragments of the two. In every 

 species of Cephalites the head is conspicuous and unmistakeable. 

 This very remarkable peculiarity is alone sufficient to distinguish 

 the genus*. 



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 noticedf. 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 J. 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 



* See ante, p. 51. 



f Ante, pp. 46, 54. It was the circumstance of the Ventriculidae being 

 polypiferous on both surfaces that rendered these provisions so necessary. 

 In Halodactylus, &c. one surface only is polypiferous. See note J p. 46. 



J 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. 



