56 THE OSTEOLOGY OF THE REPTILES 



The Skull of the Nothosauria 



(Fig. 47) 



[No manuscript. See pages 211, 246, 247] 

 The Skull of the Plesiosauria 



(Figs. 4^-48, 25 a) 



The extreme aquatic adaptations of the plesiosaurs have caused 

 certain modifications in the structure and relations of the bones of the 

 skull that are unique among reptiles. 



The general shape of the skull seems to bear a definite relation to 

 the length of the neck, always shorter in the long-necked forms, 

 slender, sometimes very slender in the short-necked types. The pre- 

 maxillae are always greatly elongated, extending back at least as far 

 as the front part of the orbit, in the long-faced kinds even to articu- 

 late with the parietals, above or between the frontals. The alveolar 

 border also is [short?] shut. The maxillae are much more elongate 

 than in the icthyosaurs and phytosaurs. There are no teeth in the 

 palate. The nasals have never been certainly determined; possibly 

 they are fused with the frontals, which extend as far forward as the 

 external nares, forming the inner border. The prefrontals are small; 

 the lacrimals have been identified in a few forms only; they appear 

 to be absent in some. The postfrontals are probably present in all. 

 The orbits are bounded below by the jugals and maxillae. The 

 quadratojugals are conceded to be absent in all plesiosaurs. The 

 single large temporal opening is bounded below by the postorbitals 

 and squamosal, above by the parietals, which are more or less ele- 

 vated in the middle in a crest. There is a pineal foramen. The 

 squamosals, in some at least, join each other back of the parietals 

 on the upper surface of the skull. There are no interparietals, 

 tabulars, or supratemporals. 



On the occipital surface the supraoccipital is excavated more or 

 less by the foramen magnum, which may extend to the roof, dividing 

 the bone. The paroccipitals are always fused with the exoccipitals. 

 The brain-case is more or less open in front on the sides, the post- 

 optics either reduced or absent. The stapes is large and stout. 



