On the Classification of the Reptilia. 



275 



opinion as to the relationship of the various orders. Certain specialisa- 

 tions in the skeleton of the hind foot and in the structure of the heart 

 and great vessels (in living forms) are of great importance in classification 

 and deserve more weight than has hitherto been attributed to them. 



The development of a hook-shaped fifth metatarsal and of a mesotarsal 

 articulation, and the sub-division of the aortic trunk so as to form 

 two systemic arches crossing at their base in such a way as to become 

 separated by the interventricular septum, clearly distinguish the Saurop- 

 sidan from the Theropsidan line of evolution. The possession of these 

 characters shows that all living Eeptilia belong to the Sauropsidan group, 

 while the structure of the foot enables us to determine the affinities of 

 many incompletely known fossil genera, and to conclude that only certain 

 extinct orders can belong to the Theropsidan branch. 



LIST OF REFERENCES. 



1. Baur, G., "On the Phylogenetic Arrangement of the Sauropsida," 'Journ. of 



Morphology,' vol. 1, 1887. 



2. Baur, G., " On the Morphology of the Vertebrate Skull," ibid., vol. 3, 1889. 



3. Baur, G., " Palteohatteria and the Proganosauria," ' Amer. Jour, of Science,' vol. 37, 



1889. 



4. Baur, G., and Case, E. C, " The History of the Pelycosauria," ' Trans. Amer. Phil. 



Soc.,' vol. 20, 1902. 



5. Boulenger, G. A., " On Some Newly Described Lizards and Rhyncho- 



cephalians," 'Ann. Mag. Nat. Hist.,' vol. 11, 1893. 



6. Boulenger, G. A., " On a Nothosaurian Reptile, etc.," ' Trans. Zool. Soc. London,' 



vol. 14, 1896. 



7. Boulenger, G. A., " On the Characters and Affinities of Telerpeton elginince," ' Proc. 



Zool. Soc.,' 1904. 



8. Broom, R., "Observations on the Structure of Mesosaurus," 'Trans. South African 



Phil. Soc.,' vol. 15, 1904. 



9. Broom, R, " A Comparison of the Permian Reptiles," ' Bull. Amer. Mus. Nat. 



Hist.,' vol. 28, 1910. 



10. Brown, B., "The Osteology of Champsosaurus," 'Mem. Amer. Mus. Nat. Hist.,' 



vol. 9, 1905. 



11. Case, E. C, "Revision of the Pelycosauria of North America," 'Carnegie Inst. 



Public.,' 55, 1907. 



12. Cope, E. D., "Vertebrate Palaeontology of Brazil," ' Proc. Amer. Phil. Soc.,' vol. 23, 



1885. 



13. Cope, E. D., " On the Homologies of the Posterior Cranial Arches in the Reptilia," 



'Trans. Amer. Phil. Soc.,' vol. 17, 1892. 



14. Credner, H., " Die Stegocephalen u. Saurier a. d. Rothliegenden des Plauen'schen 



Grundes," 'Zeit. d. Deutsch. Geo]. Ges.,' vol. 40, 1888. 



15. Greil, A., "Vergl. Anat. u. Entwickl. des Herzens," ' Morph. Jahrb.,' vol. 31, 1903. 



16. Huene, F. v., "Die Dinosaurien d. Europ. Triasf.," 'Geol. Pal. Abh. Suppl.,' vol. 1, 



1907. 



17. Huxley, T. H, 'The Anatomy of Vertebrated Animals,' 1871. 



