474 PROCEEDINGS OF THE ACADEMY OF [April, 



trinominal, l^iit on its western course, as it enters the Louisianan it be- 

 comes 0. g. sayi, and still further west, in the Texan, is even now per- 

 haps differentiating into a color form known as 0. g. splendidiis. The 

 Louisianan subspecies has moved up the Mississippi valley as far as 

 southern Illinois, and even in that region an occasional specimen shows 

 the pattern of splendidus. The lower Mississippi valley may indeed, 

 with some reason, be regarded as a secondary and more modern centre 

 of development in Chelonia and Ophidia. 



Quite different from the condition of temperature which has limited 

 the northward extension of Ocmulgian forms, that which has checked or 

 transformed them on the western route is the lack of moisture encoun- 

 tered beyond the Mississippi, about the beginning of the great plains. 



Another northwestward route from the Ocmulgian may have been 

 directly through or around the lower end of the Alleghenies by way of 

 ancient river valleys, but it is not probable that this was traveled as 

 freely as those leading by the coast plain in either direction. 



It may be repeated that the present study deals only with post- 

 glacial conditions. If, as has been assumed, the interchange of reptiles 

 between the Nearctic and the Palsearctic was by means of a north 

 Atlantic connection, their Tertiary centres of development were doubt- 

 less of greater area and much farther to the north. 



