CHAPTER X 



THE SUBCLASS SYNAPTOSAURIA 



6. ORDER SAUROPTERYGIA 



A SINGLE, large temporal opening, bounded above by the parietal, 

 below by the postorbital and squamosal. No dermosupraoccipitals, 

 tabulars, or quadratojugals. Quadrate fixed. A parietal foramen. 

 Neck elongated, the tail never long. Vertebrae platycoelous. Cer- 

 vical ribs attached exclusively to the centrum, the dorsal ribs ex- 

 clusively to the arch by a single head. A single, large coracoid on 

 each side. Girdles stout. Pelvis with large pubo-ischiatic opening, 

 or secondarily a thyroid foramen. No sternum. Parasternals stout. 



There is still much doubt as to the derivation and genealogical 

 relationships of this order of reptiles, chiefly because of the structure 

 of the temporal region. The general characters of the skeleton are 

 more or less modified by aquatic adaptations. The boundaries of 

 the temporal region seem to be those of the upper opening of the 

 diapsid reptiles; and there are many who believe that it really is 

 the upper one, and that the order is nearest related to the Progano- 

 sauria. The opening, it is seen, is bounded quite like that of some 

 members of the Therapsida, especially the Cynodontia; and these 

 reptiles are confidently believed to have descended from theromor- 

 phous reptiles with a typical lower opening. The more general 

 opinion is that the Sauropterygia are related to the anomodont-like 

 reptiles. Some, however, would trace their descent directly from the 

 Cotylosauria; others from the Diapsida, by the loss of the lower arch. 

 The author believes that the first of these views is the correct one, 

 but in the present uncertainty they may be left in an independent 

 group. 



Whatever has been their origin, we must await the discovery of 

 their more terrestrial ancestors in the early Trias. The modifica- 

 tions of structure in adaptation to aquatic life are very pronounced, 

 even in the Nothosauria. The order is clearly divisible into two 

 chief groups, the Nothosauria and the Plesiosauria. 



