AMERICAN AMPHICCELIAN CROCODILES 17 



their loss by terrestrial lizards — a fact which does not render their 

 separation from other lacertilia into a distinct suborder imperative. 

 Toads, the most terrestrial of all amphibia probably, have procoelous 

 vertebrae. 



The procoelous structure has never been attained by either 

 aquatic or highly amphibious forms; on the contrary, the amphicoe- 

 lous structure has persisted longest in such. Its attainment has 

 always been correlated with the use of the hind limbs in the support 

 or propulsion of the body upon land, and the amphicoelous structure 

 has lingered longest in the tails, as in the dinosaurs and pterodac- 

 tyls. The ichthyosaurs, with feeble hind limbs, retained a very 

 primitive structure of the vertebrae, till late in Cretaceous times, 

 until nearly all other reptiles had acquired convexo-concave vertebras ; 

 while the plesiosaurs, equally aquatic animals, and doubtless derived 

 from nearly as primitive ancestors, acquired in many cases truly 

 amphiplatyan vertebrae — a fact doubtless to be correlated with the 

 important use of the hind legs and a well-developed sacrum. The 

 procoelous vertebrae of the mosasaurs was a character acquired by 

 their ancestral terrestrial forbears, as was also their scaled skin. A 

 consideration of these facts, together with the pterygoidal inclusion 

 of the posterior nares, serving no apparently useful purpose in such 

 purely ichthyophagous reptiles, inclines me to the belief that the 

 modern gavials are late descendants of the more terrestrial proccelian 

 types such as Thoracosaurus or Holops may have been. The 

 opisthocoelous type of vertebra is of much wider distribution, and 

 has been acquired by purely aquatic animals such as Lepidosteus 

 and the true salamanders. 



No form of convexo-concave vertebrae is of profound morphologi- 

 cal significance in the evolution of the vertebrates, but I would not 

 go quite so far as does Koken in uniting proccelian and amphicoelian 

 crocodiles in the same family, notwithstanding the extraordinary 

 resemblance of other parts of the skeleton.^ 



I Koken, Zeitschrijt der Deutschen geologischen Gesellschaft, 1888, p. 768. 



