47 
of the Ventriculidje of the Chalk. 
So the individuals comprised within the description of the 
genus Cephalites exhibit two broadly-marked modifications ; the 
head of the one group being only of the same breadth as the thick- 
ness of the wall^ and being placed exactly at the top of that wall, 
and nearly at right angles, at every point, to the outer and inner 
surfaces of that wall ; the head of the other group being much 
broader than the thickness of any part of the wall, and never 
lying fiat at the top, but extending more or less down over the 
sides of the wall. These marked differences are accompanied by 
important difi'erences in the mode of fold of the membrane. I 
distinguish therefore the genus Cephalites into the two sections 
Annulatiy being those in which the head extends as a mere broad 
ring round the flat top of the wall, and Dilatati, being those in 
which it is spread out so much more extensively. 
And so also the members of the genus Brachiolites are at once 
separated into two groups, by the remarkable circumstance that 
some of them have the extremities of those projecting lobes into 
which they are divided open, others closed. The latter I distin- 
guish as the sectional division Operti, the former as Aperti. 
I shall hereafter point out the minor modifications accompa- 
nying these more striking ones, and endeavour to show the final 
purposes of the respective modifications themselves. 
It will of course be well understood that, as in every class of 
fossil forms the exact determination of the species of individual 
specimens is often difficult, frequently impossible, such must 
sometimes be the case with respect to theVentriculidse. The con- 
ditions under which they are found render them peculiarly liable 
to this difficulty ; and the inexperienced observer who has not 
yet learned to distinguish that which is a mere cast'^' from a speci- 
men in which some of the actual body is preserved, — a task of 
no slight difficulty, and only to be successfully undertaken after 
acquiring a full knowledge of structure, — will often find himself 
baffled in the attempt at specific identification. Hence the 
importance of attention to those sectional and generic characters 
already noticed, and which he will rarely be unable to distinguish. 
These broad modifications, and the respective relations thereto 
* Michelin’s Ocellaria 5^raw^^^pora,pl.40.3a&35,is amerecos/ofexternal 
and internal surfaces. The imperfection and indefiniteuess of almost all the 
figures yet published have been already noticed (ante, vol. xx. p. 78-80). It 
would therefore be a useless attempt to endeavour to identify them. Ob- 
jects of this class require to be well understood before they can be truth- 
fully represented by figures. The figures of Dr. Mantell are no exception 
to this remark, as they only give the broad external characters of one species 
(which they however do) without any indication of the mode of fold of the 
membrane which gives rise to those characters, and the very existence of 
which membrane Dr. Mantell denies. 
