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-like disc), Die- 

 tyocha, Gaillonella, Pyxidicula, and numerous kinds of the family 

 oi 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 Coscinodisci, 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 interest. An assemblage 

 of these tertiary animalculites presents so striking 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 investi- 

 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- 

 Ipng 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 prevailing organisms, show that 

 the bed consists of an aggregation of the siliceous shields or cases 

 of numerous kinds of Bacillariay but no traces of Coscinodisci or 

 other usual American tertiary species occur ; this arises probably 

 from the Irish deposit being of fluviatile origin*. 



* It 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. 



