EXTINCT ORDERS OF REPTILES. 



221 



Of the remaining genera of the Sauropterygia, Nothosaurus, 

 Simosaiorus, Placodus, Pistosaurus, and Conchiosauriis are Tri- 

 assic. In Notliosaiirus (fig. 569) the neck is long, and com- 



Fig. 569. — Skull of Nothosaurus mirabilis, reduced size. Trias (Muschelkalk), Germany. 



posed of at least twenty vertebras. The dorsal vertebrse are 

 biconcave, and the limbs are converted into swimming-pad- 

 dles. The teeth are numerous and conical, and are implanted 

 into distinct sockets. Several species are known, all Triassic, 

 and especially characteristic of the Muschelkalk. Simosaurus 

 had a large head with enormous orbits, and teeth sunk into 

 distinct sockets. This genus is exclusively confined to the 

 Muschelkalk. In Placodus (fig. 570), the teeth are in dis- 

 tinct sockets, and resemble those of many fishes in being- 

 rounded and obtuse, forming _ 

 broad crushing plates adapted 

 for the comminution of shell- 

 fish. The upper jaw contains 

 a double series of these teeth, 

 an outer or maxillary series, and 

 an internal or palatal series ; 

 but the under jaw has only a 

 single row of teeth. 



In the Jurassic period the 

 principal genus is Pliosaurus, 

 comprising huge reptiles, allied to the Plcsiosaurm in their fin- 

 like paddles, but having an enormous head supported upon a 

 short neck. The teeth are simple and conical, and in large 

 specimens attain a great size. Pliosawnis is confined to the 

 Middle and Upper Oolites. Other Jurassic genera of Saurop- 

 terygia are known, but they present no features of special 

 importance. In the Cretaceous period, lastly, the principal 

 European representative of this order is Plesiosaurus H^qH; 

 whereas in deposits of corresponding age in. North America, 



570. — Under surface of the ui'per jaw 

 in Placodus gigas. Musclielkalk. 



