REPTILIA 547 



lizards less than two feet in length are known from the strata 

 deposited in the Oligocene time in North America. Though reptiles 

 had become completely adapted to living on land, many of them 

 returned to the water. Most living turtles spend the greater part 

 of their lives in water but must return to land to deposit their eggs. 

 In the heyday of reptiles there were many other aquatic and marine 

 forms showing numerous and diverse adaptations to living in a 

 liquid medium. The ichthyosaurs (fish lizards) were the most per- 

 fectly adapted to aquatic life, being very fishlike in form. 



CLASSIFICATION OF LIVING REPTILES 



Many different plans have been proposed for grouping the reptiles. 

 All of these plans have attempted to express the relationships of 

 the various groups to each other; and the relations of representa- 

 tives of a group to other members of the same group. The following 

 arrangement is adapted from Williston, and includes only those 

 groups having living representatives. 



Class — Reptilia 



Subclass — Anapsida. Temporal region of skull not perforated. 



Order — T estiidvnata. A single coracoid; ten dorsal vertebrae, their ribs 

 expanded to meet on the dorsum or a dermal layer of bony plates. 

 Turtles. 



Suborder — Fleurodira. Neck retracted laterally; pelvis united with the 



plastron. 



No representatives of this suborder are found in North America. 

 Suborder — Cryptodira. Neck retracted vertically; pelvis not united with 



the plastron; carapace with marginal plates. 



The majority of the living turtles of North America belong to thi? 



suborder. 



Suborder — Trionychoidea. Neck retracted vertically; carapace with no 

 marginal plates. 



The ''soft-shelled" turtles make up this suborder. 

 Subclass — Parapsida. Temporal region of skull with one opening. 



Order — Squamata. Quadrate freely articulated proximally (streptostylic) 

 or secondarily fixed. Lizards and Snakes. 

 Suborder — Lacertilia (Sauria). Parietals never united to basisphenoid 

 by descending plates; the brain case more or less membranous an- 

 teriorly. 



The one hundred and seventy-four kinds of lizards known from the 

 United States and Lower California belong to this suborder. 



