of the Ventriculidse 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 



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



I size had been attained, individuals dead or destroyed below that 



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



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



I however, without considerable hesitation. 



; The whole genus Cephalites is characteristic of the Middle 



t 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 



I Lower Chalk. 

 Hbi, § a. Annulatif, 



HHpead narrow and flat : plaits compact and regular. 



7ephalites longitudinalis. PI. VII. (vol. xx.) fig. 1, & PI. XIV. 



' 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 



^^noderately thick. 



PP^This species much resembles in external aspect the smaller 

 f 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 

 I alone is sufficient to distinguish the two at a glance. 



This is the only species of Cephalites in which the longitudinal 

 [ ibid 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 



^wain 



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



^ Mag. N. Hist. Ser. 2. Vol.i. 19 



