84. Dr. G. A. Mantell on the Animalculites of the 
before this Society (but not published), the dark vems and mark- 
ings in the pillars of Purbeck marble in the Temple Church, 
are attributable to the remains of the soft bodies of the fresh- 
water shells of which that limestone is composed, in the state of 
molluskite. 
I have stated my conviction that the experienced microscopical 
observer will not hesitate to agree with me in the opinion, that 
in the fossils before us we have the mineralized soft bodies of 
polythalamia; and I have obtained, through the kindness of 
Mr. Williamson of Manchester, a recent object for comparison, 
which is perfectly analogous, not to say identical, with the best- 
preserved flint specimen. It is the body ofa Rotalia from which 
the shell is removed, and is associated with other polythalamia, 
&c.; 1t was obtained ‘with numerous other interesting recent or- 
eanisms in sediment from the Levant. 
Infusoria in Flint :—Xanthidia.—Our flints contain abundance 
of several kinds of infusoria; as for example, various species of 
the genera Pywidicula, Peridinium and Xanthidium. 1 shall re- 
strict my remarks to the last-named animalculites, which, from 
their elegant forms and good state of preservation, are highly in- 
teresting to microscopic observers. The Xanthidia are minute, 
globular or spherical bodies (from 535th to =45th of an inch in 
diameter), beset with tubular processes, which terminate either 
in fimbriated or acuminated extremities. They are stated by 
Ehrenberg to be siliceous, and to be analogous, and some of the 
species identical, with living forms which abound in boggy pools 
and ponds. Several of the recent kinds occur in the ponds on 
Clapham Common, Hampstead Heath, and other ‘places around 
London. These organisms are however considered by the most 
eminent botanists not to belong to the animal kingdom, but to 
be vegetable structures, related to the Desmidiacee; and are 
defined as plants having “ fronds simple, constricted mn the mid- 
dle ; segments shghtly compressed, turgid, reniform or orbicular 
and entire; their surfaces more or less furnished with simple or 
brancbed elongated spines, either scattered over the surface or 
confined to the margin, where they are placed in two rows, one 
on each side the marginal line*.” Ehrenberg, on the other hand, 
describes the Xanthidia as animals having spontaneous motion 
and increasing by self-division. But I must not dwell on this 
important and difficult question ; the arguments on both sides 
ave concisely stated in ‘ Annals of Nat. Hist.” March 1845, p. 188, 
to which I would refer those who are interested in the subject. 
I do not presume to think that my opinion on this problem is 
of any value ; but waving the question of the animal or vegetable 
* See Mr. Ralfs’s paper on the Desmidiacea, Ann, Nat. Hist. Jan. 1845. 
