VERTEBRATA; REPTILIA 953 



tailed and sometimes scaly. The Squauiata comprise the lizards 

 and snakes and two extinct groups of aquatic reptiles from the 

 Cretacic (Mosasaurus, etc.). The lizards (Lacertilia) have 1,925 

 living species but few fossil ones are known, the oldest being from 

 the late Jurassic. Of the snakes (Opliidia) nearly 1,800 recent 

 species but only about 35 fossil ones are known, chiefly from the 

 Tertiary, though some Cretacic forms are probably referable to 

 snakes. The IchtJiyosauria are entirely extinct reptiles which in- 

 liabited the Triassic, Jurassic, and Cretacic seas. Their body was 

 in general whale- or fish-like and the jaws were furnished with nu- 

 merous conical teeth. 



The Sauropterygia, also restricted to the Mesozoic, were mostly 

 marine, lizard-like reptiles, with long necks and well-developed 

 limbs, with five normal digits {Nothosaiiridcc Triassic) or paddle- 

 shaped, the digits elongated by supernumerary phalanges (Plesio- 

 sauridcc, Trias to Cretacic). The Therouwrpha were primitive land 

 reptiles with many mammalian characters and often of grotesque 

 forms and proportions (Pareiasaurus, Dicynodon, etc.). They 

 lived in the Permic and the Triassic of North America, Europe, 

 and South Africa. 



The Chelonians or turtles are characterized by the possession 

 of a more or less complete bony shell, partly composed of modified 

 neural spines of the dorsal vertebrse and partly of dermal ossifica- 

 tions more or less intimately united with the former. The limbs, 

 tail, and generally the neck and head can be withdrawn into this 

 shell. In general a dorsal shield, or carapace, and a ventral one, 

 or plastron, composes this shell and both are, as a rule, superficially 

 covered by a horny or leathery epidermal layer divided by grooves 

 or sutures into a few large scutes or shields. Their arrangement 

 is independent of the underlying osseous plates. Turtles first ap- 

 peared in the Upper Triassic (Keuper) of Europe. 



The Crocodilia are lizard-like reptiles with the highest internal 

 organization of the class. Their skeletal structure differs widely 

 from that of lizards, and their respiratory organs resemble those 

 of birds. The entire body is covered with horny scales. The most 

 primitive groups {Parasuchia), resembling the Rhynchocephalia, 

 occur in the Trias of America (Belodon or Phytosaurus, and Epis- 

 coposaurus) ; of Scotland (Stagonolepis) ; and the Gondwana 

 formation of India (Parasuchus). There are also more specialized 

 Triassic forms, such as the little Aetosaurus (of which 24 complete 

 individuals occur on a single block of Stuben-sandstone [Upper 

 Keuper] in the Stuttgart Museum), and others from the Trias of 

 Elgin, Scotland. 



