220 Mr. Toulmin Smith on the Ventriculidse of the Chalk. 



The descriptions which have been given of the mode of fold 

 on the inner surface of V. decurrens and V. radiatus render it 

 unnecessary to dwell on the nature of the modifications which 

 the present species exhibits, and which are but the reduplication, 

 of the remarkable characters exhibited in the species just named. 

 It may be suggested, however, that sections of this species will 

 vary so much, according to the direction in which they are taken, 

 that no one figure can be depended on as certainly indicative of 

 the species, without very extended and careful comparison. Not- 

 withstanding, however, the complexity of the folding in the pre- 

 sent species, the application of the observations already made in 

 respect of V. decurrens will make it clear that the free access of 

 sea-water would be perfectly maintained to all parts of the very 

 extended poljrpiferous surface which is thus gained. 



In all these deeply folded species we have seen that the move- 

 able processes are very conspicuous. They are thus conspicuous 

 on that portion of the fold which is most exposed. There would 

 obviously not be room for their operation within the folds, and 

 the traces of them are not found there as they are on the external 

 surface. If their object be that which has been suggested*, their 

 presence and action on this exposed external surface would be 

 amply sufficient for the protection of the polyps in the deeper 

 recesses of the folds, and the presence of the processes there 

 would be useless. The absence of their marked development in 

 those parts is therefore in harmony with that admirable adapta- 

 tion of means to ends which the results of every part of the pre- 

 sent investigation have displayed. 



In examining the various and complicated forms which have 

 been thus noticed as included within the genus Ventriculites y 

 nothing is more striking than to find that, be the wall of the 

 pouch thick or thin, the thickness of the membrane itself remains 

 always the same. It may be folded up in the most complicated 

 way, but it still retains, in any single piece of it, precisely the 

 same characters as have been described as typical in respect to 

 V. simplex. Were other facts wanting, this would alone go 

 far to induce the conclusion which other courses of investigation 

 have already seemed to demonstrate, viz. that we have before us 

 a true Polyzoic polypidom ; and that polypidom one of the most 

 admirable construction and contrivance. No less interesting 

 illustrations of that conclusion will be found in the widely differ- 

 ent forms and modifications of folding which remain to be exa- 

 mined. 



* Ante, p. 205, and p. 185 of vol. xx. 



[To be continued.] 



