296 Evolution and Distribution of Fishes 



not cause us to presuppose a large lake of such extent, nor 

 even a contemporaneous freshwater connection. But one 

 must accept that over this region at the same or at succes- 

 sive times, wide freshwater connections existed, that were 

 taken advantage of in the progressive migrations of these 

 fishes. 



Though no great importance need meanwhile be at- 

 tached to the identification by Newberry of specimens from 

 near Liege, Belgium, which he considered to be specifically 

 the same as his Dinichthys tuberciilatus, such a view at 

 least suggests much for future verification geographically. 

 The geologic zones in which the American arthrodires 

 occur extend from the Delaware limestone — not from the 

 marine Columbus proper as some have asserted — up 

 through the Huron and Cleveland shales (Fig. 43, p. 269), 

 or their equivalents in surrounding states, such as the Che- 

 mung and Catskills of Pennsylvania and New York. As 

 already pointed out these are (p. 268) freshwater deposits, 

 and if at times intercalated strata show marine organisms 

 the arthrodires are there conspicuous by their absence. But 

 such a record as "Arthrodire fragments, Encrinal lime- 

 stone (Hamilton) ; Eighteen mile creek, New York" could 

 readily be explained as a washout by the sea of some fresh- 

 water strata, and succeeding redeposits of the fragments 

 in that sea alongside marine shore organisms. 



The climax in size, abundance, and variety of species 

 was reached in the top zone or Cleveland Shale of Ohio, 

 and the Genesee-Portage shale of New York, both black 

 bituminous shales that have yielded abundant freshwater 

 elasmobranch fishes, true dipnoans and ganoids, but 

 which is entirely destitute of any assemblage of marine 

 organisms. The continuation westward of these beds into 

 Iowa and Wisconsin greatly extends the range of the 

 Arthrodires, but also raises important environal questions. 

 Thus in some of the earlier volumes of the Iowa Geological 

 Survey it is repeatedly stated that the abundant though 

 broken and rounded teeth and bones that make up a breccia- 

 conglomerate from three inches to three feet In thickness 

 belongs to the Cedar Creek formation, and the statement 

 is also reiterated that there are, mixed with the fish remains. 



