of the Ventriculidae of the Chalk. 281 



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

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

 dwelt*, 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. Annulatif. 

 Head narrow and flat : plaits compact and regular. 



1. Cephalites longitudinalis. PI. VII. (vol. xx.) fig. 1, &P1 XIV 



fig.l. 



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 

 the wall thicker. The depressions on the inside also are generally 

 smaller, closer, and more regular than in that species. The head 

 alone is sufficient to distinguish the two at a glance. 



This is the only species of Cephalites in which the longitudinal 

 fold remains unmodified on the outer face. Hence its specific 

 name. A transverse section of it is seen on fig. 1 of PI. VII. 



It is a rare and delicate species : indeed all the species of the" 

 present genus are rare. They do not seem to have abounded in 

 the older seas of the Middle Chalk as the Ventriculites did in the 

 Upper Chalk. Though thus rare, however, their modifications are 

 not the less clearly marked. 



In regard to the head it is proper to remark, that while, 

 throughout the present division of this genus, its breadth will 

 always be found a very near approximation to that of a transverse 

 section of the plaits, there is a slight variation in this respect in 

 individual specimens. The head often slopes a little outwards, so 



* See ante, p. 204. f See ante, p. 47. 



Ann. fy Mag. N. Hist. Ser. 2. Vol. i. 19 



