of the Ventriculidse of the Chalk. 213 



been based. The inquirer will be well-prepared for this entire 

 absence of any such tubuli from its having been seen that, in the 

 preceding species, the depressions (which alone are what could, 

 even on a superficial observation, be called tubuli) have the 

 greatest diversity of form and character, a diversity which at 

 once negatives their being of the nature assigned to them by 

 Dr. Mantell. It has moreover been seen that, as a matter of fact, 

 they are in no instance tubuli, but that they can, on the contrary, 

 be traced as folds of the membrane forming the polypidom itself. 



On the other hand, the present is the only species which could 

 seem to lend any support to the contractile theory. The observer 

 however who has studied the several preceding species will pro- 

 bably ask for no proof that the present does not stand out as an 

 anomaly and an exception from them all. If he need such, he 

 may be referred to the observations already made *, generally, on 

 the matter. These have full application in this as in every other 

 case, and sufficiently show that this species offers no violation to 

 that Unity which prevails through every branch of the present 

 inquiry. 



Different specimens vary in the size and depth of the plait. 

 It is suggestive, perhaps, as before, of difference of age. It is 

 where the plait is deepest and broadest that the moveable pro- 

 cesses become most conspicuous. 



The plaits are not narrow at the base and of increasing size 

 as they approach the margin. They maintain the same size from 

 base to margin, and the increase of surface is effected by the 

 increase in number of plaits as the margin is approached ; an in- 

 crease effected by the division, from time to time, of one plait 

 into two, — each of equal size with the original one, — a mode of 

 increase which generally prevails in all species in which the plait 

 can be distinctly followed. The species is not common. 



§ b. Complicati. 

 Inner and outer surfaces not corresponding. 



1. Ventriculites mammillaris. PI. XIII. figs. 7 & 14. 



Inner plaits simple and regular : outer plaits raised in large hol- 

 low bosses at regular intervals : processes very conspicuous : 

 thickness of wall considerable. 



This is the first instance in which we find the direction of the 

 fold changed between the external and internal surfaces. There 

 is no species more strongly marked, and none which affords a 

 better illustration of the value and importance of a principle of 

 Unity as the essential and most valuable means to true scientific 



* Ante, vol. xx. p. SO. 



