of the Ventriculidse of the Chalk. 47 



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 flat at the top, but extending more or less down over the 

 sides of the wall. These marked differences are accompanied by 

 important differences in the mode of fold of the membrane. I 

 distinguish therefore the genus Cephalites into the two sections 

 Annulati, 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 opeUf 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 the Ventriculidse. 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 gr andipora,ii\AO. 3 a ^3h, is a mere cast of external 

 and internal surfaces. The imperfection and indefiniteness ofalmostallthe 

 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. 



