O. C. Marsh— Vertebrate Life in America. 345 
character of most of the rocks then formed, which were not 
well fitted for preserving such remains, although admirably 
adapted to retain foot-prints. 
During the Cretaceous Period, Reptilian life in America 
attained its greatest development, and the sediments laid down 
in the open seas and estuaries were usually most favorable for 
the preservation of a faithful record of its various phases. With 
out such a perfect matrix as some of these deposits afford, many 
of the most interesting vertebrates recently brought to light 
from this formation would probably have remained unknown. 
The vast extent of these beds ensures, moreover, many future 
discoveries of interest. 
Some of the earliest forms are allied to the modern genus 
Trionyx. In the higher Cretaceous beds, some Chelonians of 
enormous size have been found. They belong to the genus 
T ery numerous, an 
with them are many fresh-water forms of Jrionyx and allied 
genera, 
A striking feature of the American Cretaceous fauna, as con- 
trasted with that of Europe, is the almost entire absence in our 
strata of species of Jchthyosaurus and Plesiosaurus, whic 
abound in many other regions, but here seem to be replaced by 
