of the Ventriculidse of the Chalk. 289 



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 pj„ q 



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. compressus, 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. B-it 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. 



