ROTIFERA. 41 



gastric Infusoria at remote periods of the history of the earth ; and 

 they are much more extensive, and will be more durable, than the 

 proudest mausolea by which Egyptian kings have endeavoured to 

 perpetuate the memory of their existence. 



In another point of view the Polygastric Infusoria are highly re- 

 markable. Their extremely minute size, simplicity of structure, 

 tenacity of life, and extraordinary powers of reproduction, have 

 enabled them to survive, as species, those destroying causes which 

 have exterminated all the higher forms of animals. Several species, 

 for example, still exist, which were in being at the period of the de- 

 position of the chalk, and which contributed their silicious remains to 

 the flinty masses which are always more or less intermixed with cre- 

 taceous matter. Before this discovery no remains of higher-organised 

 animals at present in existence had been detected, with the same degree 

 of certainty, in the cretaceous formation. A few existing zoophytes 

 and testacea first make their appearance in the tertiary beds immediately 

 above the clialk ; hence called, by Mr. Lyell, Eocene, from eoe, the 

 dawn, as indicating the fii'st dawn of the creation of existing species. 

 The number of existing species of shells increases in the Miocene, 

 and is still greater in the Pliocene tertiary strata ; but the higher 

 animals, as the Anoplotheria, Palcpotheria, Mastodons, Mammoths, 

 and other mammalian contemporaries of the Eocene, Miocene, or 

 Pliocene testacea, have utterly perished. The discovery, therefore, 

 by Ehrenberg, of several, at least twenty, species of silicious shelled 

 Infusoria, fossil, in the chalk and chalk marls, which are perfectly 

 identical with those from the sands of the Baltic and North Sea, is a 

 most interesting addition to the obscure history of the introduction of 

 the successive species of animals on this planet, and must add greatly 

 to the interest of this Infusorial class in the eyes of the naturalist and 

 geologist. " For these animalcules,'' says Ehrenberg, " constitute a 

 chain, which, though in the individual it be mici'oscopic, yet in the 

 mass is a mighty one, connecting the organic life of distant ages of the 

 earth, and proving that the dawn of the organic nature coexistent 

 with us reaches farther back in the history of the earth than had 

 hitherto been suspected." 



The still existing species are by no means rare or isolated, but fill 

 ill incalculable numbers the seas of Northern Europe, and are not 

 wanting on the tropical coasts of the globe. With reference to 

 the operations of the invisible Polygastria at the present day on 

 tiiese and other coasts, I have only time to refer you to the translation 

 of a late paper by the indefatigable Berlin professor, entitled, " Ob- 

 servations upon the important Part which Microscopic Organisms 

 play in the choking up of the Harbours of Wismar and Pillau ; 



