The most successful of Permian land animals were the peculiar reptiles that learned to live in drier regions. Like the horned toad and Gila monster of our arid southwestern United States, the larger Permian reptiles were four-footed animals. In size and shape they were not greatly different from amphibians then living. An exception to this rule, among some of them, is the development of long, bony spines above the vertebrae of the back. A fairly common fossil of this type, found in Texas and known as _Dimetrodon_, had a total length of six feet, about half of this being in the tail. The tips of the spines adorning the back reached a height of three feet or more and there was probably a covering of skin over these bones, which would produce a sail-like structure or “fin” of large size. Its use has not been explained but it provides an easy name for these odd creatures—the “fin-back lizards.” Rock deposits produced in arid regions usually have characters which are not difficult to recognize. Gritty texture, irregular bedding, red color, and gypsum are common features. Formations of Permian age are to be found in Colorado but better fossil deposits have been discovered in Kansas and Texas. [Illustration: Marine Reptiles: Plesiosaur (Lower Skeleton) and Mosasaur The Mesozoic Era produced many types of reptiles besides the dinosaurs. Two of the marine forms are shown in this illustration, both from Cretaceous beds of the western United States. Plesiosaurs were the giants of the seas in their time, lengths of forty to fifty feet being not uncommon. A long flat tail provided locomotion for the mosasaurs whereas the plesiosaurs resorted to the peculiar limb structures known as flippers or paddles.] THE AGE OF REPTILES The Mesozoic, or era of middle life, was a long stretch of time during which there was marvelous development among the reptiles. Many strange types were produced and most of them became extinct before the end of the era. The reptilian stock branched out in many directions. Types emerged which differed from one another so widely that their mutual relationships have become obscure. Hideous and fantastic creatures suggesting sea serpents and dragons were worldwide in distribution. Reptiles of the air and seas acquired large size and weird forms, but greater advances were made upon land.