55 



sion of the whole mass and relative size of the central cavity are 

 usually greater than in any other genus, thus compensating for 

 the frequently looser nature of the fold in the other genera. It 

 may perhaps be inferred that the ocean in which this genus 

 dwelt was subject to such modifications that its waters were less 

 liable to disturbance than those of the ocean in which Cepha- 

 lites and Brachiolites originated ; so that a larger size could be 

 attained in safety, and with less multiplied provision for secu- 

 rity against such disturbances. 



The whole genus Ventriculites is characteristic of the upper 

 chalk. I have a few specimens which may possibly have come 

 from the middle chalk* ; but every such case is doubtful, while 

 I have carefully ascertained that the mass are certainly from the 

 upper chalk. 



a. Simplices. 



Inner and outer surfaces corresponding. 



1. Ventriculites simplex. PL VIII. fig. If. 

 Membrane simple and without trace of fold : moveable processes 

 minute or absent. 



This is the type of the whole family, and it is a most happy 

 circumstance that it exists. That existence at once destroys all 

 the theories as to the anatomy and physiology of this group of 

 fossils which have been suggested from examination of some spe- 

 cial forms only ; while it affords the means of demonstrating some 

 important points. It appears to be a rare species, as, though I 

 am fortunate in having myself obtained a very perfect series, I 

 have certainly never seen half a dozen specimens out of my own 

 cabinet. 



When the underskin and polyp-skin are absent, as most often 

 happens, the specimen, if uninjured, is particularly beautiful. It 

 exhibits, over its whole surface, with the utmost regularity, that 

 square arrangement of fibre, with the subtending octahedra, already 

 described. When the underskin is present without the polyp- 

 skin the squares are not seen, but the whole surface is covered 

 with a much smaller-meshed tissue which appears dotted all 

 over, the dots being the polyp-cells. When the polyp-skin is 

 present it hides each of these appearances, and is marked only 



* See Ann. and Mag. Nat. Hist. vol. xx. p. 337. 



t It is not possible to give illustrations, in this work, of all specimens of 

 full size, several of them often exceeding in diameter the size of the page. 

 It will be understood, therefore, that all the specimens of the present genus 

 figured on PI. XIII., with the exception of fig. 9, are of specimens below the 

 usual size. 



