'1'1\ The American Geologist. October, 1894 
the coast through the growth of the coastal plain would 
tend to Increase the temperature of the water on the new 
soundings, as W. F. Ganong* has noted as the effects of mod- 
erate uplift in recent geological times. Shoaling and straight- 
ening out of the shore-line diminish the tides, and these being 
less do not so much perturb through the admixture of the 
deeper cold water the surface waters warmed by the sun. 
With the development of the coast shelf synchronously with 
the peneplain there was brought into existence a population 
of marine reptiles, the mosasaur and its allies. Just as land 
mammals, as we first know them, are paralleled by representa- 
tive marine cetacean species, so, in the abundance of reptil- 
ian life, forms were adapted to existence in the sea and on the 
and 
The Life inhabiting the Peneplain. 
Turning now to the more familiar account of the fauna and 
flora of the Mesozoic, we find both of these productions at the 
apparent opening of the era remarkably dissimilar to the Pal- 
eozoic forms. During the succeeding secondary periods 
equally noteworthy changes took place. Along with this 
rapid advance in the higher vertebrate ranks, we have the 
seeming fixity in some of the invertebrates after the comple- 
tion of the peneplain. 
Tin freshwater mollusca attained their present characteristics 
before the close of i he Cretaceous. 
We have to note that the freshwater mollusca of North 
America attained approximately their present characters dur- 
ing the period of evolution of the baseleveled lowlands, and 
that they have undergone little modification in the succeeding 
periods until now. "To so great a degree had this differenti- 
ation then attained.*' states Dr. ('. A. White in his report on 
the Cretaceous invertebrates of the Plateau Province, "that 
the species of Dnio, Helix, Physa, etc., seem to have been as 
diversified and well developed as they are at the present time. 
Indeed the species of these genera are so closely like some of 
those now living that the}' need only the fresh condition of 
recent shells to remove all suspicion of their great antiquity 
from the mind of the casual observer.*' One of these genera. 
♦Southern Invertebrates on the. Shores of Acadia. Trans. Roy. Sec. 
Canada, ? iv. 1890, (h>. 167-185) p. 1st. 
