FOSSIL VERTEBRATES — REPTILIA 725 
TESTUDINATA. 
The turtles may be briefly described as reptiles that have lost 
the teeth completely, and have developed an external skeleton 
in the shape of a bony case that is more or less complete and 
protects the body of the animal. There is little doubt that they 
are derived from animals that did not possess such a structure, 
but the point of their divergence from the primitive stem is so 
ancient that it has not yet been detected, and the oldest form 
that we know is a member of the most specialized group of the 
turtles. 
The order is generally divided into three suborders: 
Trionychia. 
Pleurodira. 
Cryptodira. 
The TZryonicdia differ from the typical forms of the Zestu- 
dinata in that the bony exoskeleton is largely represented by a 
thick, leathery skin, which has given them the name of Soft-shell 
Turtles. Beneath this skin is developed a series of small bony 
plates with a characteristic sculpture, that in many places form 
the chief remains of the forms. 
Trionyx is the most common of the extinct forms. The old- 
est remains are from the Upper Cretaceous of New Jersey; these 
are, for the most part, fragmentary remains of the plates of the 
carapace. From the Upper Cretaceous, Laramie, of the western 
part of the United States and British America, have been col- 
lected a large number of specimens. Through all the deposits 
of Tertiary age in Europe, specimens of this genus have been 
found; in the United States it is found throughout the Tertiary 
of the eastern part, but in the west seems to be confined to the 
lower layer, the Eocene. Remains of still living species have 
been found in the Pleistocene beds of India and Burmah. 
The Cryptodira are by far the most important group of the 
Testudinata, both in number of species and their distribution in 
time. The group is characterized by the fact that the pelvic 
bones are not anchylosed to the carapace and the plastron above 
and below. There were developed so many forms that it is 
