324 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18. 



Fusconaia undata rubiginosa 

 Pleurobema obliquum coccineiim 

 Pleurobema clava 

 Ptychobranchus phaseolus 

 Obovaria circulas lens 



Another one, Symphynota costata, should be added, since, 

 although the genus is found in the east, the subgenera are different 

 (Lasmigona and Symphynota). 



This shows that although a number of the typical western genera 

 have gone way up into the headwaters, they have not been able to 

 cross the divide. 



III. On the Atlantic side (see list no. 23) we have two genera 

 (Margaritana and Lexingtonia) , which are not found in the interior 

 basin. Margaritana has, indeed, a related form (Cumberlandia 

 monodonta (Say)) in the Tennessee and Ohio drainage, but there 

 is probably no direct genetic connection between them, and the his- 

 tory of Margaritana, as will be seen below, is a case by itself. 



IV. Lexingtonia is possibly related to and descended from cer- 

 tain interior basin forms (such as Fusconaia and Pleurobema), but 

 the relationship is remote, and for all practical purposes we may 

 class it with the cases to be mentioned presently. These are the 

 following forms (of list no. 23) : nos. 3, 4, 5, 6 (the four species of 

 Elliptio), and nos. 10, it, 12 (Alasmidonta collina, heterodon, un- 

 dulata). All these are forms of the respective genera, which have 

 no closely allied or representative forms on the western side, although 

 the genera are represented there. 



Attention should be called to the fact that Lexingtonia, three 

 species of Elliptio (fisherianus, productus, lanceolatus) and Alasmi- 

 donta collina undoubtedly belong to the southern element in the At- 

 lantic fauna, and that their distribution northward is limited. How- 

 ever, it is also probable that Elliptio complanatus, Alasmidonta 

 heterodon and undulata belong to the same class. The first and 

 third are undoubtedly southern in their affinities, and allied species 

 are frequent upon the southern portion of the Atlantic slope (in 

 the Carolinas and Georgia). This is not so clear in the case of 

 Alasmidonta heterodon. Here it has the appearance, as if the dis- 



