73 



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 t, 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 

 that, probably, the ocean in which this genus dwelt being, appa- 

 rently, a more disturbed one than that in which the Ventriculites 

 dwelt J, and the head possibly not forming till a certain age and 

 size had been attained, individuals dead or destroyed below that 

 age very rapidly lost their form and are therefore found only as 

 shapeless masses. I do not suggest this solution of the difficulty, 

 however, without considerable hesitation. 



The whole genus Cephalites is characteristic of the Middle 

 Chalk. I have never found a single specimen which I could with 

 any probability refer to the Upper Chalk, though it may be ex- 

 pected that some forms will be found which endured into that 

 later epoch. Certainly none have been ever yet found in the 

 Lower Chalk. 



a. Annulati . 

 Head narrow and flat : plaits compact and regular. 



1. Cephalites longitudinalis. PL VII. fig. 1, & PI. XIV. fig. 1. 



Plaits delicate but often deep : outer plaits slightly winding : 

 inner plaits depressed at short and regular intervals ; bulging 

 on each side around depressions till the adjoining plaits meet 

 and open into each other : processes very conspicuous : wall 

 moderately thick. 



This species much resembles in external aspect the smaller 

 cylindrical specimens of Ventriculites tenuiplicatus. It is however 

 smaller than that species usually is, the plaits less winding, and 



* 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. campanidatus and C. constrictus. 



f As to structure and nature of this see ante, pp. 24, 37. 



J See ante, p. 55. See ante, p. 52. 



