84 ' 



This is seen in the following curious section of a flint of this 

 species in mv possession. 



& 1 * i re 1 Flff. K. 



bucn cases only afford 

 instances of the care and 

 caution necessary in the 

 investigation of such a 

 subject as the present. 



It will be clearly seen 

 that the mode of fold, of 

 which an attempt has 

 thus been made to convey an idea, secured free access of sea- 

 water to all parts of the surface of the membrane, external and 

 internal. By the same contrivance that membrane was held se- 

 curely in its position ; the regular funnel-shape assumed by the 

 last plait, with its margin fixed at the top, securing it within * ; 

 the campanulate envelope, to which the projecting points of the 

 plaits were affixed, securing it without. The size of the internal 

 cavity varies in different specimens, as will be seen by figs. 12 

 and 13 of PL XIV. ; of which fig. 12 is a general view of the 

 external aspect of one specimen ; fig. 13 is a section, with the 

 matrix cleared away from the inner funnel-shaped simple mem- 

 brane, of another. 



There is certainly no form among the Ventriculidse which 

 might, at first sight, be less supposed than the present to have 

 had any affinity with the fossils which have been described as 

 belonging to the genus Ventriculites. This will be well under- 

 stood by comparing PL XIV. figs. 12 and 13 with any of the 

 figures on PL XIII. 



The condition in which these specimens are found, their deep 

 folds preventing their ever coming free from the chalk, or being 

 developed without the laborious use of the needle, renders it 

 impossible to make any confident observations upon them as to 

 the processes ; a remark which also applies to every other species 

 of the present section. 



3. Cephalites constrictus. PL XV. fig. 1. 



Whole body very low, much-elongated and narrow, with roots at 

 one end : plaits very deep, and running longitudinally from the 

 root extremity; each plait constricted at short and not very 

 regular intervals, and sometimes to nearly its whole depth : 

 cephalic membrane covering the whole upper surface and sides, 

 to the margins of which last, as well as to many of the pro- 

 minent points of the plaits, it is attached; usually constricted 



* See observations on the head, hefore, p. 74. 



