of the Veiitriculidse of the Chalk. 

 Fig. H. Fig. I. 



291 



In each figure the dotted line is the envelope. Fig. H is a 

 longitudinal section as it would be seen if one could be taken 

 exactly clear through any spot where there was no lateral divi- 

 sion of a plait — the presence of which gives a false appearance of 

 anastomosis to a section. It will be seen that the membrane 

 may be traced from the point of the base to c and thence to a 

 in a continuous line, and that the projecting plaits are affixed, at 

 various points, to the envelope a — b. Fig. I is a transverse sec- 

 tion taken about the middle of a specimen. The inner circle is 

 the membrane where simple and unwaving, and forming there- 

 fore necessarily an unbroken circle. Between this and the en- 

 velope the plaits are seen, cut horizontally across. 



The membrane begins to fold upwards in its last plait at dif- 

 ferent distances in different specimens from the inner margin of 

 the head ; and it is rarely that the folding upwards will take place 

 on exactly the same plane all around ; whence, on section, a de- 

 ceptive appearance is often given, as if there were a double or 

 triple or still more numerous ramification of the central cavity. 

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

 species in my possession. 

 Such 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 * ; 

 * See observations on the head, before, p. 282. 



