M. K. A. Zittel on Fossil Hexactinellida. 423 



the Eocene sandstone of Brussels*, and from Miocene sands 

 at Ruditz, in Moravia, and of a Miocene AphrocalUstes from 

 Russia, I know of no Tertiary Hexactinellida from Northern 

 and Central Europe. 



But even in the South-European Nummulitic formations, 

 to which, at least in part, a deep-water origin is ascribed, it 

 is remarkable that they occur only as great rarities. The 

 only certain evidence of them consists in an Eocene Guet- 

 tardia, which D'Archiac has described from the neighbom'hood 

 of Biarritz. 



This hitherto unexplained gap, however, is partially filled 

 by A. Pomel's important discovery of numerous Miocene 

 sponges in the province of Oran. Among the North-African 

 Hexactinellida the genus Craticularia {Laocoetis^ Pom.) takes 

 the first place by its astonishing abundance of forms : species 

 of AphrocalUstes {Badinskia, Pom.), Tretostamma^ Pom., 

 and Placochlcenia, Pom., as well as a considerable number of 

 Lithistida, are also described. 



If, therefore, the fossil Hexactinellida, by their peculiar 

 geological distribution, prove to be deep-sea dwellers almost 

 as clearly as their living relations, we obtain in these orga- 

 nisms an important datum for judging of the mode of forma- 

 tion of geological deposits. 



The limitation of the fossil Hexactinellida to deep-sea 

 deposits, however, also necessarily involves their intermittent 

 occurrence, separated by long interruptions. In formations 

 which are at present known only under a littoral facies, there 

 are no Hexactinellida. The different sponge-horizons are 

 therefore also in part separated by enormous intervals of time. 

 Thus, for example, the Silurian forms are immediately fol- 

 lowed by the Upper Jurassic (no true spongitic deposits are 

 known in the Devonian, Carboniferous, and l)yas) ; and these 

 again are separated by a wide interval from the Middle and 

 Upper Cretaceous. This best explains the fundamental dif- 

 ferences in the successive sponge-faunas in the Silurian, the 

 Jura, the Cretaceous, and the Miocene. Under these cir- 

 cumstances we ought rather to wonder that tliere are any 

 genera common to two formations, than that the Jurassic and 

 Cretaceous Hexactinellida, for example, present great differ- 

 ences. 



Perhaps there are few divisions of the animal kingdom, 

 capable of ])rcservation, of the phylogeny of which paU\!onto- 

 logy furnishes so fragmentary a picture. Our entire know- 

 ledge of the fossil Hexactinellida is limited to isolated and 



* Kuloi, /. r. j)l. iii. Hga. .'j.'i, 'M. 



