514 Evolution and Distribution of Fishes 



Switzerland, in Bohemia, in N. America, in Sumatra and 

 elsewhere, show oil shales with piled together fish remains, 

 that are renowned as petroleum producers. These beds 

 also often include or adjoin strata with land plants, insects, 

 frogs, tortoises, etc. Heer's graphic description of the 

 Oeningen locality is specially noted, and from his accounts 

 it seems likely that the deposit is a volcanic dust. 



Comparative tables of the freshwater and marine fishes 

 of these beds are then presented. 



Chapter 9. The Primitive Fishes in time and space. 



Review is next made of the occurrence and relation of 

 the primitive fishes. The author divides these, as well as 

 the higher fishes, into three groups, the Malacodermata, 

 the Placodermata, and the Lepidodermata. The first in- 

 cludes the Cyclostomata and Palaeospondylida. To the 

 former of these the writer would asign many "genera" of 

 the conodonts found in rocks from Silurian to Carbonifer- 

 ous age. They occur usually side by side with other and 

 higher fish remains from Russia to N. America, and from 

 their often teeming abundance indicate a prolific conodont 

 fauna.. Reasons are given for regarding many of these as 

 true cyclostome teeth. From its structure Palaeospondylus 

 is placed near the Cyclostomata. 



Account is next taken of the types that make up the 

 seven subdivisions of the Placodermata, and their destruc- 

 tion in time and space is traced. Evidences are advanced 

 for viewing all of the ancestral forms of these as lake, 

 river, or marsh dwellers. The seventh subgroup of Micro- 

 placoda or Elasmobranchii is viewed as probably deriva- 

 tive from genera like Birkenia or Lasanius. Types like 

 or related to these seem to have evolved the Acanthodii, 

 the Cladoselachei, and the Pleuracanthii, of all of which 

 however the remains often consist only of teeth, spines, 

 or scales. The included genera were all freshwater, and 

 persisted more or less from Lower Old Red to Permian 

 days. Their remains occur also alongside the giant arth- 

 rodires and other cuirassed fishes in Old Red Strata. Evi- 

 dence is again advanced in support of the view that in early 

 Carboniferous time a considerable seaward migration of 



