M. K. A. Zittel on Fossil Lithistidw. 245 



occur in great quantities, and the HexactincllidiB only very 

 sparingly. This indicates that the conditions of existence of 

 the two sponge-groups were similar but not identical. In 

 general the living Lithistidas appear never to occur at such 

 great depths as the Hexactinellida3. Deposits in which the 

 latter predominate must therefore have been formed in deeper 

 water than the formations which abound in Lithistidge. 



In decidedly littoral deposits we know scarcely any Lithis- 

 tidas ; and this circumstance may chiefly serve to explain the 

 evidently very imperfect materials in our palff-ontological 

 museums. They are limited to isolated deposits, widely sepa- 

 rated both in space and time, and each as a rule containing 

 a different sponge-fauna from those of the earlier- or later- 

 formed strata. We cannot demonstrate any constant and gradual 

 evolution of the fossil Lithistidse ; nay, even the passage of 

 a genus, to say nothing of a species, from one formation 

 into a later one can hardly be proved. 



The Palaeozoic formations have hitherto furnished only a 

 single well-defined genus* (Aulocopium) . 



I know of no Lithistidai from the Trias and Lias ; but I 

 have found a Melonella and a species of Cnemidiastrum in the 

 Hoheneo-o-er collection from the Brown Jura of the Cracow 

 district. 



The Lithistidge are remarkably numerous, especially in indi- 

 viduals, in the Sponge-limestones of the White Jura. Here 

 the genera Cnemidiastrum^ Hyalotragos^ Flatychonia^ and 

 Cylindrophyma especially predominate. Leiodorella, Episto- 

 mella, Pyrgochonia, Discostroma, Megalithista^ LecaneUcij 

 Mastosia, and Melonella occur more isolatedly. In the Lower 

 White Jura they occur in moderate numbers along with the 

 Hexactinellidte which there predominate ; but their chief de- 

 velopment only appears in the White Jura 2, in which the 

 Hexactinellidse are rather more scanty. In the older horizons 

 of the Cretaceous formation the Lithistidge are not entirely 

 deficient, but they occur only as rarities. On the other hand, 

 the Cenomanian is the home of numerous species of Siphoma, 

 Jerea, StichojyJiyma, Chonella, Verruculina, and Amphithelion. 

 Celebrated localities are tlie Greensands of Blackdown and 

 Haldon, the Glauconitic Chalk of Normandy and Touraine, 

 and the Lower Planer of North Germany, Saxony, Bohemia, 

 and Bavaria. 



* Possibly tlie insufRciently described genera Aulocopina, CalatMum, 

 and Euspongin of Billings also belong to the LithistidtB. The Silurian 

 genera Archceocyathus and Trachium of Billings require to be micro- 

 scopically examined in order to ascertain whether they are really to be 

 referred to the Hexaotinellidre. 



