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 
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. Annulati^. 
Head narrow and flat : plaits compact and regular. 
1. Cephalites longitudinalis. PI. VII. (vol. xx.) fig. I, & PI. XIV. 
fig. I. 
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. ^ Mag. N. Hist. Ser. 2. Vol. i. 19 
