Baseleveling and Organic Evolution. — Woodworth. '2'2-i 
Physa, has been found in the ancient freshwater fauna of the 
Lower Coal Measure limestone of the Eureka district in Ne- 
vada,* a discovery which shows for how long a time the ex- 
isting mollusca of the land waters have held their own on the 
American continent. In what has been said concerning the 
mutations of the drainage of the inland waters, the attempt 
has been made to point out the manner in which their differ- 
entiation may have been brought about. These mollusca 
survived the geographic changes which at the close of the 
Mesozoic destroyed the function of the peneplain as a low- 
land ; in this respect the mollusca differ widely from the rep- 
tilian group, which, as we shall see, pass out as the peneplain 
was uplifted and dissected. 
Changes of the fishes and amphibians not well understood in 
relation to the peneplain of this date. 
While it is among the vertebrates that the most obvious al- 
terations are found, not all of the changes are readily traced 
to the bearing which the lowlands of the Mesozoic might have 
had upon their particular groups. Among fishes the ganoids 
held sway as late as the culmination of the peneplain in Cre 
taceous times, but they gave way then to the teleosts and have 
now but a scanty representation in our rivers. That there 
was a marked change in the character of the fishes in known 
areas at this time is well known, but how far the differentia- 
tion of the fishes had been accomplished through these river 
changes is difficult to determine, particularly for the reasons 
that freshwater fishes are rarely found, and that these, like 
the mollusca, are invariably lacustrine species. We must be 
content with pointing out, in this place, the manner in which 
the river changes may have operated to modify the species. 
Of amphibians, the labyrinthodonts, the first of which ap- 
peared as early. as the Carboniferous, survived the Appala- 
chian revolution, attained their grandeur in the Trias, and 
apparently disappeared in North America before the Upper 
Cretaceous. The exact cause of their decline is probably to 
be sought in the development of the more powerful reptilia. 
*A. Hague, Geology of the Eureka District, Monog. \\. 1 . S. Gool. 
Survey. 1892, |». 87. 
