. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 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
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 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 a
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