ICHTHYOPTEKYGIA. 219 



Vertebral centra, ossified, biconcave ; joined by syndes- 

 mosis, not by suture, to their neural arch. Pleura- 

 pophyses of the trunk long and bent, the anterior ones 

 with bifurcate heads. Teeth with converging folds of 

 cement at their base ; implanted in a common alveolar 

 groove, and confined to the maxillary, premaxillary, 

 and premandibular bones. Premaxillaries much exceed- 

 ing the maxillaries in size. Orbit very large ; a circle 

 of sclerotic plates. Nostrils near the orbits. Limbs 

 natatory ; with more than five multi-articulate digits. 

 An episternum and clavicles. No sacrum. 



With the retention of characters which indicate, as in the 

 preceding orders, an affinity to the higher Ganoidei, the pre- 

 sent exclusively marine Eeptilia more directly exemplify 

 the Ichthyic type in the proportions of the premaxillary and 

 maxillary bones ; in the shortness (fig. 92, c) and great number 

 of the biconcave vertebras ; in the length of the pleurapophyses 

 of the vertebras near the head ; in the large proportional size 

 of the eyeball with its well-ossified sclerotic coat ; and especi- 

 ally in the structure of the pectoral and ventral fins. 



It has been usual to unite the present with the following- 

 order in the same group, called Enaliosauria or sea-lizards. 

 Both were adapted for marine life, but breathed the air like 

 the Cetacea : they were, however, " cold-blooded," or of a low 

 temperature, like crocodiles and other reptiles. The proof 

 that the Enaliosaurs respired atmospheric air immediately, 

 and did not breathe water by means of gills like fishes, is 

 afforded by the absence of the bony framework of the gill- 

 apparatus, by the presence, position, and structure of the 

 air-passages leading from the nostrils to the mouth, and 

 by the bony mechanism of the capacious chest or thoracic- 

 abdominal cavity ; all of which characters have been demon- 

 strated by their fossil skeletons. With these characters the 



