Chalk and Flint of the South-east of England. 85 



nature of these bodies, I may be permitted to state, that a careful 

 examination of both recent and fossil Xanthidia leads me to doubt 

 whether there is any analogy whatever between the organisms in 

 our flints and their supposed living types. The fossil forms have 

 the body more decidedly spherical or globular, their spines more 

 strictly tubular and differently arranged ; and they never exhibit 

 that reniform or constricted character so constant in the recent 

 Xanthidia, nor do they present any indication of spontaneo.us 

 fissuration. The fossils are supposed by Ehrenberg to have been 

 originally siliceous like the shields of other infusoria, but I know 

 not that any proof has been obtained of this inference. On the 

 contrary, so many examples occur in which the tubular arms are 

 bent, contorted, and contracted and shrivelled in the middle, 

 as to convey the idea of a flexible, rather than of a brittle, un- 

 yielding substance. A crushed or torn specimen very lately found 

 by my son, exhibits an appearance much at variance with the 

 supposition that the original was composed of silex or of any 

 other material that had a conchoidal fracture. If the Xanthidia 

 were originally siliceous, there is no reason why they should not 

 be detected in the chalk itself, since bodies equally minute are 

 readily discoverable*. If to these arguments be added the a-priori 

 objection as to the probability that inhabitants of fresh water, of 

 boggy pools and ponds, should be found swarming in the sponges 

 and other marine structures of the cretaceous ocean, I think in 

 the present state of our knowledge it will be proper, notwith- 

 standing the high authority from which we must differ, to con- 

 sider the so-called Xanthidia of the chalk as distinct from the 

 recent organisms after which they have been named ; in fact, as 

 a genus of marine infusoria, should they not hereafter prove to 

 be the gemmules of polyparia or the spores of marine plants. 



I will conclude this imperfect notice of the flint animalculites, 

 by stating that several kinds of disciform bodies of great beauty 

 have recently been detected by Mr. Lee; these appear to be 

 transverse sections of different species of the foraminifera termed 

 Nodosaria, or of some allied genus. 



III. Tertiary Animalculites. — I now arrive at the last division 

 of the present inquiry, which will comprise a few remarks on the 

 occurrence of animalculites in the tertiary strata of Great Britain ; 

 and of living species and genera of infusoria in the British seas, 

 analogous to those of the miocene deposits of North America. 



The organic constitution of the tertiary marls of Virginia, and 

 the nature of the fossils of which they are composed, are too well 



* Since the above remarks were written, numerous Xanthidia have been 

 detected in chalk from Dover by Mr. Henry Deane of Clapham; but the ap- 

 pearance of these specimens, when cleared from the chalk and mounted in 

 Canada balsam, seems to support the opinion that the originals were flexible 

 and not siliceous. 



