I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE 341 



SphceriidcF, the Anciilosa dilatata-carinata group of the Pleuroceridce, 

 Campeloma decisum, and the crayfishes, Cambarus bartoni and 

 longitlns (I disregard, for the present, C. spinosus and C. acumi- 

 natus, as probably belonging to the Tennessee-Coosa problem, at 

 any rate to a region lying to the south of the one which interests 

 us here). 



These forms generally go way up into the mountains, and prac- 

 tically meet there with the western range of the respective forms, so 

 that the distribution seems almost continuous across the mountains, 

 and suggests crossing of the divide. 



There is great variety in the details of distribution of these 

 forms, and two main groups may be distinguished : those with a 

 more universal range on either side of the mountains, and those with 

 a more restricted range on one or on both sides. 



The above is a sketch of the chief distributional features, and 

 we see that it is possible to group a number of cases under the same 

 heads, which means to say that very likely similar causes have acted 

 to bring about similar distribution. But before we begin the task 

 to investigate the laws which governed these different types of dis- 

 tribution, it is necessary to recall to our mind certain fundamental 

 facts with regard to the physiography of the Alleghenies. 



Chapter 5. 

 Physiographical Facts. History of the Allegheny Moun- 

 tain Region. 



The origin and the development of the Appalachian or Alle- 

 ghenian mountain system is rather well worked out (see McGee, 

 1888, Davis, 1889, Davis, 1891, Willis, 1896, Hayes, 1896, Davis, 

 1907), and we may assume that its general features are established. 

 We do not need to go much into detail here, but certain phases in 

 the mountain forming process should be brought out, which will be 

 important for our present purpose. 



