43 2 CROCODILIA CHAP. 



an anterior or ventral single copulatory organ can of course be 

 asserted of recent forms only. 



In spite of these many characters common to all Crocodilia, 

 it is very difficult to separate the latter from the Dinosauria, the 

 only absolute difference lying in the ventral pelvic bones. It 

 is therefore most suggestive that the fore-limbs of the Mesozoic 

 Crocodilia are so much shorter and weaker than their hind- 

 limbs, a discrepancy which is not lessened before the Tertiary 

 epoch. The Mesozoic Crocodilia were almost entirely marine; 

 the strongly-developed ankle-joint (indicated already by such 

 early forms as Aetosaurus and Mystriosaurus) must have been 

 inherited from some terrestrial group with digitigrade tendencies 

 and shortened hind-limbs. All this points to some Theropodous 

 Dinosaurian stock of which the Crocodilia may well form an 

 aquatic, further - developed branch. Loss of the pubic and 

 ischiadic ventral symphysis is not a serious modification. So 

 far as modern reptiles are concerned only the Chelonia and 

 Sphenodon are related to the Crocodilia, whilst Monitors and 

 other lizards resemble them only superficially. We divide them 

 into three Orders. 



Order I. PSEUDOSUCHIA. 



The few members of this peculiar group of reptiles are all 

 restricted to the Keuper or variegated marls, although they 

 seem to have had a wide distribution, some having been found 

 in Germany, others in New Mexico. They perhaps form av. 

 early side-branch of the generalised Crocodilian stock, which died 

 out with the Jurassic age. 



The skull is distinctly short and pointed. The premaxil- 

 laries are very small and are dorsally separated from each other 

 by the large nasals, which also keep the maxillae widely asunder. 

 The nostrils are latero-terminal, bordered chiefly by the nasals, 

 below by the premaxillae and part of the maxillae. The orbit 

 is bordered below by the strong jugals, in front by the prefrontal, 

 above by a supra-orbital and a small postfrontal, behind by a 

 postorbital, which, firmly connected with the jugal and squamosal, 

 shuts off a supratemporal foramen. There is also a lateral 

 temporal fossa, and a large hole enclosed by the lacrymal and 

 the maxillary bones. The teeth are restricted to the anterior 



