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 spontaneous 
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, simce bodies equally minute are 
readily discoverable*. If to these arguments be added the @-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 flit 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. 
Ill. 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. 
