Mr. Toulmin Smith on the Ventriculidse of the Chalk. 279 



chyma, it appears to me that the laws of gi-owth of leaves must 

 be looked for in the course of the development of their frame- 

 work, the nerves. These are apparently organized gradually out 

 from the stem into the nascent leaves, just as the vascular bundles 

 into the apex of the stem, and their point of separation in the 

 blade being fixed from the first, it is clear that all growth in the 

 blade of the leaf must occur beyond this, and it is most natural 

 to suppose that the nerves become organized from this centre 

 outward as the vascular bundles were from the stem at first. 

 Thus it would happen that Dicotyledonous leaves in general 

 would grow at their base until they were sketched out as it were, 

 in the bud, but as soon as the nerves were formed and the plan 

 of the framework of the future expanded lamina laid down, the 

 growth would be apical, marginal and interstitial. In Monoco- 

 tyledonous leaves with straight veins there appears to be nothing 

 to prevent the continued development of the base, and as we 

 usually find the tissue in a softer and less consolidated condition 

 there, it is probable that that part is the seat of development. 

 These ideas are merely suggested as rational interpretations of 

 the facts before us, but much systematic observation is required 

 before this question can be settled. 



XXIX. — On the Ventriculidse of the Chalk; their classification. 

 By J. Toulmin Smith, Esq. 



[Continued from p. 220.] 



Genus Cephalites. 



Character. Pouch-shaped : very constant in size and dilatation : 

 cavity usually regular and with a single opening ; sometimes 

 winding and with more openings than one : membrane form- 

 ing the wall of the cavity always deeply folded : marginal 

 edges — and, sometimes, most prominent points — of the plaits 

 attached to a simple apolypiferous membrane stretched across 

 their whole breadth and forming the upper margin or head of 

 the wall : membrane of wall polypiferous on both external and 

 internal surfaces. 



The differences between the genera Cephalites and Ventriculites 

 are so broadly marked that, except in one or two species, it would 

 be difficult to confound even fragments of the two. In every 

 species of Cephalites the head is conspicuous and unmistakeable. 

 This very remarkable peculiarity is alone sufficient to distinguish 

 the genus*. 



* See ante, p. 46. 



