324 ORTMANN— THE ALLEGHENIAN DIVIDE. [April i8, 



Fusconaia iindata rubiginosa 

 Pleurohema ohliqunm coccincum 

 Pleurohema clava 

 Ptycliohrancluis phascolns 

 Ohovaria ciraihis Icus 



Another one, Syiiiphynota costafa, should be added, since, 

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

 (Lasmigona and Sytnphynota). 



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 {Cnmberlandia 

 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 Plenrobema) , 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 (Alasinidonta collina, lieterodon, iin- 

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

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

 the genera are represented there. 



Attention should be called to the fact that Lexingtonia, three 

 species of Elliptio {fislicrianns, 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 complanatns, Alasmidonta 

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

 third are undoubtedly southern in their afihnities, 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- 



