1904,] NATURA.L SCIENCES OF PHILADELPHIA. 473 



Species, H. platyrhinos, is found upon suitable soils over most of the 

 Atlantic subregion and is especially common on the coast from New 

 Jersey to Florida; H. simus is Austroriparian and chiefly Ocmulgian; 

 while the smallest and most feeble species only, H. nasicus, is Sonoran 

 and does not extend west of the Rocky mountains. There is good 

 reason, therefore, to refer Heterodon to the Austroriparian, in which 

 case its Neotropical affinity renders it probable that Florida was the 

 region of its origin. 



Among Crotalida^, Ancistrodon only can be referred to the Atlantic 

 subregion, A. contortrix being spread over its whole extent and A. 

 piscivorus confined to the Austroriparian, where numerically it is in 

 excess in the extreme southeast. Both species enter the borders 

 of the Sonoran at suitable localities. The assignment of the genus 

 to the Atlantic coast is further justified by its occurrence in eastern 



Asia. 



Of these more or less cosmopolitan, or Holarctic genera, which must 

 be supposed to have occupied the Nearctic during the Tertiary, and 

 even of related groups restricted to the Nearctic but of sufficient 

 extension there to warrant belief in their relatively considerable age, 

 it is seen that there is reason to believe that all but Sceloporus, Cnemi- 

 dophorus, Eumeces, Sistrurus and Crotalus belonged to the Atlantic 

 post-glacial centre, which from the present numerical proportion of 

 species and individuals, as well as the geological age of the region, 

 seems to have been the Ocmulgian. 



From this centre a route to the north has been freely open along the 

 low coast plain east of the Alleghenies, to distances into the Eastern 

 district determined for each migrant by limiting conditions, of which, 

 in this case, temperature must be regarded as the most important. 

 A few powerful species, such as Coluber obsoletus obsoletus, Ophibolus 

 doliatus triangulus and to a less extent PityopMs melanoleucus, have 

 chosen this route to the region north of the Ohio river, turning west- 

 ward about the Potomac or Susquehanna. 



The other main highway was westward along the Gulf coast to the 

 Mississippi river and up that valley, where conditions are highly 

 favorable to reptilian life, spreading eastward at the north to southern 

 Indiana and Ohio. Few species traveled this route unaltered, for with 

 most of them the changed conditions encountered have resulted in 

 the establishing of variations, at least of color, with which a sound 

 taxonomy must reckon. An excellent example of this is Ophibolus 

 getulus getulus, which ranges from Florida north to the pine region of 

 New Jersey with no change which has yet been held to warrant a new 



