AND ON THE EVOLUTION OF THE CROCODILIA. 429 



Parasucliia, in those respects in which they differ from the Meso- 

 suchia, approach the Orniihoscelida and Lacertilia, especially such 

 Lacertilia with amphiccelous vertebral centra as the existing Hat* 

 teria and the extinct Hyperodapedon, the affinities of which I havo 

 elsewhere indicated*. In fact I know of no other reptile in which 

 the skull and pectoral arch so nearly approach the structure found 

 in Belodon and Btagonolepis as they do in Hatteria. On the other 

 hand, the Eusuchia are those Crocodilia which depart most widely 

 from the Ornithoscelida and Lacertilia, and are the most Crocodilian 

 of Crocodiles. 



The differences between the Mesosuchia and the Eusuchia are of 

 no great moment. The MM. Deslongchamps, in the remarkable 

 memoir to which I have referred, show that in Metriorhynchus, 

 which ranges from the Kelloway rock to the Kimmeridge clay, the 

 secondary posterior nares are carried further back than in Pelago- 

 saurus, which is confined to the Upper Lias. Let the pterygoids of 

 Metriorhynchus begin to unite, and let the posterior faces of the 

 centra of its vertebrae, instead of being a little concave, become a 

 little convex, and it would furnish a perfect transitional form between 

 the Mesosuchia and the Eusuchia. 



On the other hand, the Parasuchia, in the conformation of the 

 posterior and the position of the anterior nares ; in the non-enclosure 

 of any part of the Eustachian canals by bone; and in the con- 

 figuration of the coracoid, depart much more widely from the 

 Mesosuchia, closely connected with the latter as they are by their 

 dermal armour and in other ways. 



From a purely morphological point of view, then, these three sub- 

 orders of the Crocodilia form a series, slightly interrupted between 

 the Eusuchia and the Mesosuchia, but with a larger gap between the 

 latter and the Parasuchia. But whatever the value of the breaks 

 between the three terms of the series., it is clear that the modi- 

 fications which are needed to connect one term with another are 

 of the simplest kind, and are throughout of the same order. The 

 kind of change which would convert a Parasuchian Crocodile into a 

 Mesosuchian, would, if continued, convert a Mesosuchian into a 

 Eusuchian. Hence, if there is any valid historical foundation for the 

 doctrine of evolution, the Eusuchia ought to have been developed 

 from the Mesosuchia, and these from the Parasuchia; and if this 

 process of evolution has taken place under such conditions that the 

 skeletons of the Crocodilia which have been subject thereto have 

 been preserved, geological evidence should show that the Parasuchia 

 have preceded the Mesosuchia, and the Mesosuchia the Eusuchia, in 

 order of time. 



Now this is exactly what the geological evidence does prove. 

 It is established that these reptiles appear in the following order : — 



* " On Hyper odapedon," Quart. Journ. Geol. Soc. 1869, p. 147. 



