340 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



interior basin (except Margaritana) . A certain uniformity of this 

 fauna is also expressed in two ways : 



(a) By the uniform and wide distribution of certain species, 

 indicating the possibility of intermigration between the various river 

 systems ; 



(b) by the fact that the fauna of each river, disregarding a few 

 lowland species, goes up, in its bulk, into the mountains and ap- 

 proaches closely the headwaters without appreciable depauperation. 



2. There is a differentiation of elements within the Atlantic 

 fauna, indicating different origin. 



(a) A southern element pointing to a secondary center of radia- 

 tion in the southern parts of the Atlantic slope is distinguishable. 

 This center itself, however, lies chiefly outside of the region dis- 

 cussed here. Forms like Lexingtonia, like those of the Elliptio 

 complanatus and fisherianus-group, Alasmidonta collina, heterodon, 

 and undulata, Eurynia constricta, among the Najades, Goniobasis 

 virginica and symmetrica among the Pleuroceridce, Cambarus bland- 

 ingi and acuminatus, among the crayfishes, belong here. These 

 forms exhibit morphologically the greatest independence, and are 

 possibly the oldest element in the Atlantic fauna. In some cases 

 it is hard or impossible to connect them with types of the interior 

 basin by more than general relationship. 9 



(b) In the northern section of the Atlantic slope exists a group 

 of forms, which are more closely related to species of the interior 

 basin and often must be regarded as their direct representatives. 

 These are the Najades enumerated under group V. (p. 325), and the 

 crayfish, Cambarus limosus. They all have their main range in the 

 north, and toward the south they disappear sooner or later, and have 

 no representatives in the south. Very often their southward range 

 becomes restricted to the coastal plain. 



(c) Further, there is a third group among the Atlantic forms. 

 These are either conspecific with western forms or extremely closely 

 allied. These are the Najades mentioned under VI. (p. 325), the 



9 It might be mentioned here, that these forms probably will be intimately 

 connected with the Tennessee-Coosa problem, and their number will be greatly 

 added to, when the fauna of the Carolinas and of Georgia is taken into con- 

 sideration. 



