444 EVOLUTIONARY MORPHOLOGY 



derive the Sauropsidan type of aorta from the mammalian, or 

 vice versa, and it is necessary to go back to a primitive type 

 like that of the amphibia where the aorta is not only undivided 

 but the pulmonary arch has not yet become separated off. 

 The primitive Cotylosaurs may have been of this type. It is 

 certain that the Synapsida must have resembled the mammal 

 (for the latter was derived from the former), and therefore 

 differed from the Sauropsida as regards the structure of the 

 aortic arches. This is the same divergence which appeared 

 from a consideration of the hook-shaped 5th metatarsal. 



Literature 



Goodrich, E. S. On the Classification of the Reptilia. Proceedings of the 

 Royal Society, Ser. B, vol. 89, 191 6. 



Nopcsa, F. Die Familien der Reptilien. Fortschritte der Geologie und 

 Palaeontologie, 2, 1923. 



Watson, D. M. S. On Seymouria, the Most Primitive known Reptile. 

 Proceedings of the Zoological Society, London, 1918. 



