289 
of the Ventriculidse of the Chalk. 
inner and outer surfaces : central cavity small : head rounding 
and very wide : wall falling in very rapidly but in a regular 
slope from outer margin of head to root : diameter of whole 
body greater than its height. 
In some specimens of the present species the plaits are very 
traceable on the outside ; in others much less so^ on account of 
the almost total absence of oxide of iron. In each case, however, 
it is equally obvious that the number of plaits seen on the outer 
surface is given by the longitudinal division and reduplication of 
the plaits towards that surface, in the same 
way as the increase of plaits from base to margin 
has already been described as being effected 
by a transverse division and reduplication*. 
The accompanying figure will explain the pre- 
sent mode of this reduplication. This arrange- 
ment takes place to some extent in most of 
the Annulati, but the very small size of the 
central cavity in C. capitatus renders this peculiarity constant in 
this species, and one of its most marked characteristics. 
The general form of this species is so peculiar that a vertical 
section through the fossil displays a triangular figure, of which 
the base of the fossil forms an obtuse angle, while the external 
margins of the head form acute angles with the wall. It is thus 
impossible to confound this species with C. compressusy as the 
wall of that species, like that of every other species of the sec- 
tion Annulati, usually forms, inside and outside, nearly a right 
angle with the headf. 
2. Cephalites campanulatus. PI. XIV. figs. 12 & 13. 
Plaits very deep ; increasing very rapidly from base and dividing, 
and so reduplicating, very constantly, both longitudinally and 
transversely ; after attaining the fullest expansion, folding 
inwards and downwards, and gradually contracting till they 
more or less nearly approach the base, whence, folded upwards 
in a single plait, the membrane rises, usually simple and 
plain, in a funnel form, to the margin of the head surrounding 
the central cavity, to which its marginal edge is attached : head 
enwrapping the body and attached to all the prominent plaits 
as far as the point where they incline rapidly towards the base : 
diameter of body greater than height. 
* See ante, p. 213. 
f I have an interesting specimen of this species in which two individuals 
are close together ; actually touching. But they cannot be mistaken for an 
example of C. constrictus, each individual having separate roots or places of 
roots, (see before, p. 46,) and not being parts of one single body. 
Fig. G. 
