222 GEOLOGICAL DISTRIBUTION. 



the proportion among the Cretaceous species for the same region is 

 fully the equivalent of seventeen per cent. In the Miocene the 

 number of such equivalent forms appears to be still further lessened. 

 We find that among the Australian Tertiary Mollusca by far the 

 greater number of species are forms which have been for the first 

 time described from that region, although the European contingent 

 of the Cretaceous fauna is by no means an inconsiderable one. 



Climatic Zones. In a very ingenious paper on the Jurassic 

 fauna, 76 Professor Neumayr, of Vienna, following the views that 

 had already been expressed by Trautschold and Marcou, has at- 

 tempted to show that climatic zones were already well differentiated 

 in the Jurassic period of geological time, and that homoiozoic belts, 

 corresponding to these, existed then pretty much as they do now. 

 Thus, on the Eurasiatic continent he recognises three distinct Jurassic 

 zones, a "Boreal," a "Central European," and a "Mediterranean" 

 the first comprising the region of Northern and Central Russia, 

 from the Petchora to Moscow, and the deposits of Spitzbergen and 

 Greenland; the second, the region lying north of the Alps, with 

 France, England, Germany, and the Baltic provinces ; and the 

 third, the region of the Pyrenees, Alps, and Carpathians, with a 

 probable continuation in the Crirneo- Caucasian belt. The faunas 

 of these different regions are most intimately related to one an- 

 other, so much so that at first sight they would appear to form a 

 homogeneous whole. But Professor Neumayr has shown that, de- 

 spite this apparent homogeneousness, there are certain well-marked 

 differences which impress a distinct individuality on each of the 

 three. Thus, as one of the most distinctive biological characters 

 of the Mediterranean zone, we have the great development among 

 the Cephalopoda of the ammouitic forms belonging to the groups 

 (or genera) Lytoceras and Phylloceras, which are but feebly repre- 

 sented in the deposits of the Central European zone, and whose 

 substitutes there are the genera Oppelia and Aspidoceras. Both 

 these series of forms are wanting in the boreal zone, which also 

 lacks the coral-reef structures of the last. Inasmuch as the differ- 

 ences here noted occur in regions which appear to have been in 

 open-water communication with each other, it is contended that 

 the absence from one fauna of the forms most distinctive of the 

 other could only have been conditioned by direct climatic in- 

 fluences. 



