REPTILIA. 211 



ent day these forms are all geographically restricted in their 

 range, and are never all associated together. It is also a 

 singular fact that the genus Gavialis, now entirely Asiatic in 

 its distribution, should occur in the Nortli American area in 

 deposits as old as the Eocene Tertiary. 



The Amphicmlian Crocodiles are characterised by their 

 biconcave vertebrae, and are entirely extinct, being confined 

 altogether to the Mesozoic period. The biconcave vertebrae 

 show a decided approach to the structure of the backbone 

 in fishes ; and as the rocks in which they occur are mostly 

 marine, there can be little doubt but that these Crocodiles 

 were, in the majority of cases at any rate, inhabitants of the 

 sea. Others, however, which are found in fresh-water de- 

 posits or estuarine accumulations, probably resembled the 

 existing Crocodiles in principally frequenting rivers. From 

 a palseontological point of view the Amphicoelian Crocodiles 

 are much the most important group of the order Crococlilia, 

 as they are, also, its most ancient representatives. 



The earliest types of the Amphicmlia, and therefore of the 

 order Crococlilia, are the Stagonolepis and Belodon of the 

 Trias. The affinities of the former genus 

 have been worked out by Prof. Huxley, who 

 has shown that Stagonolcjns resembled the 

 existing Caimans in general form, but that it 

 possessed the elongated skull of the Gavials. 

 The body was protected by a ventral and dor- 

 sal series of bony, pitted scutes, but there were 

 only two rows of the latter ; and the teeth 

 have obtusely -pointed crowns, which some- 

 times show signs of having been subjected 



T T. 7 7 1 111 Fig- 5t51.- Tooth 



to attrition. In Belodon, on the other hand, ot Beiodon caronn- 

 the teeth (fig. 561) are long, pointed, and Zitn'^t'' '''"''' 

 conical, slightly curved, and longitudinally 

 striated. The jaws were greatly elongated, as in the exist- 

 ing Gavials. 



In the Jurassic series the remains of Amphicoelian Croc- 

 odiles are abundant, and belong to such genera as Steneosaurus, 

 Tclcoscmrus, Makrosjpondylus, Pclagosaurus, Mctriorhyncliiis, &c. 

 The first two of the forms mentioned above are the most 



