306 CLASSIFICATION OF VERTEBRATES. 



SUB-ORDER 3. PLACODONTIA. 



Palatine teeth large, pavement-like ; premaxilla with incisors, maxilla 

 with rounded molars ; lower jaw with incisors and pavement teeth ; one oc- 

 cipital condyle. The rest of the skeleton is unknown. Placodus, European 

 trias. 



SUB-ORDER 4. THERIODONTIA (PELYCOSAURIA). 



Teeth differentiated into incisors, canines, and molars ; intercentra fre- 

 quently present ; supra- and infratemporal fossae developed ; two or three 

 sacral vertebrae ; carnivorous. Clepsydrops, Permian of Texas and Illinois; 

 Dimetrodon and Naosaurus from the Permian of Texas, both with enormous 

 spinous processes ; in the latter these bear several transverse bars ; Gale- 

 saurus, trias of Africa. 



ORDER II. PLESIOSAURIA (SAUROPTERYGII). 



Extinct aquatic reptiles, apparently with naked skin ; the 

 tail short, the neck long ; a single occipital condyle ; temporal 

 fossa present ; teeth in alveoli, quadrate immovable ; anterior 

 nares separate, near orbit ; a parasphenoid sometimes pres- 

 ent ; no sclerotic ring in orbit ; vertebrae amphicoelous or flat ; 

 ribs with a single head ; abdominal ribs present ; sternum and 



FIG. 302. Restoration of Plesiosanrus, after Dames. 



precoracoid absent, the coracoids meeting in the middle line ; 

 feet pentadactyl and usually modified into swimming-organs. 

 The plesiosaurs were large carnivorous reptiles, sometimes 

 reaching a length of forty feet. In Nothosaurus and Lario- 

 saurus the feet were fitted for creeping, and the animal was 

 lizard-like ; triassic of Europe. In Plesiosaurus the limbs were 

 flipper-like, the phalanges being greatly increased in number, 

 while the neck was extremely long. Allied genera are Cimolio- 



