86 Dr. G. A. Mantell on the Animalculites of the 
known to require particular description. They are almost entirely 
made up of the aggregated siliceous cases or skeletons of infusorial 
animalcules ; the prevailing forms belonging to the genera Cos- 
cinodiscus (sieve-like disc), Actinocyclus (wheel-hke disc), Dic- 
tyocha, Gaillonella, Pyxidicula, and numerous kinds of the family 
of Bacillaria. Figures and descriptions of many of these fossils by 
Dr. Bailey will be found in several of the late numbers of the 
‘American Journal of Science.” The most remarkable of the si- 
liceous shields are the orbicular cases of the Coscinodisct, which, 
when entire, consist of a pair of discs, connected at the periphery 
by a broad band or ring. The delicate and elegant markings with 
which the surfaces of these shields are elaborately sculptured, 
render them objects of great beauty and mterest. An assemblage 
of these tertiary animalculites presents so strikmg a contrast to 
any I have seen from the chalk of England, Asia, or America, that 
I am very desirous M. Ehrenberg’s statement as to their preva- 
lence in cretaceous strata should be verified by further ivesti- 
gations ; and the more so, as Dr. Bailey mentions that Ehrenberg 
referred certain unquestionably miocene American deposits to the 
chalk, because they yielded animalculites resembling some he had 
obtained from European strata supposed to belong to the Chalk 
formation. 
I have sought in vain among the tertiary strata of England 
for infusorial deposits analogous to those of America. Polytha- 
lamia frequently occur in the London clay (as was first made 
known by Mr. Wetherell in a valuable paper published in our 
Transactions); and within the last few weeks several kinds of fo- 
raminifera have been obtained from clay brought up in sinking a 
well at Clapham, at the depth of 120 feet. But no one has dis- 
covered in our tertiary formations a bed, or even seam of earth, 
composed of fossil infusoria. In fact, so far as my information 
extends, our only rich deposits of this kind are of very recent ori- 
gin. Near the banks of the river Bann, in the county of Down, 
Ireland, there is a layer of infusorial earth a foot thick, under- 
lying a bed of peat. Specimens of this earth, with which I have 
been favoured by the Countess of Caledon, accompanied with 
drawings by her ladyship of the prevailmg organisms, show that 
the bed consists of an aggregation of the siliceous shields or cases 
of numerous kinds of Bacillaria, but no traces of Coscinodisci or 
other usual American tertiary species occur ; this arises probably 
from the Irish deposit being of fluviatile origin*. 
* Tt may be added, that the property of polishing metal, which deposits 
of this kind are so well known in Germany to possess, has been discovered 
by the Irish ; and as this earth occurs on the estate of Lord Roden at Tul- 
leymore, it is locally known as Lord Roden’s plate-powder. 
Some white earth recently sent from New Zealand as magnesia, proves 
to be a fluviatile infusorial deposit like that of Ireland. 
