I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 369 



Of course, this presupposes that the original home of this form 

 was in the interior drainage basin. But I hardly think that this 

 could have been otherwise, on account of the tremenduous range 

 of Strophitus edentulus in the west, and we have seen that the 

 Atlantic slope probably never has been an important center of de- 

 velopment. Strophitus differs from all the other elements of the 

 Atlantic fauna (discussed so far) by the fact that the identical 

 species is found on either side of the mountains. Thus it is improb- 

 able that it had a similar history to that of the other forms (the 

 northern and southern elements) of the Atlantic fauna, and we are 

 forced to assume a special explanation of its distribution. I think, 

 that the evidence introduced above favors the theory, that it actually 

 crossed the divide by the help of stream capture, or in other words, 

 by the shifting of the divide, and that this probably took place in 

 the region of the headwaters of the West Branch Susquehanna. It 

 might have happened elsewhere ; it might have happened repeatedly : 

 but the region indicated is the most likely. After having once (or 

 repeatedly) crossed, this species spread over the Atlantic slope, both 

 north and south, and occupies now the whole of it, from Virginia to 

 New England (exact data from Virginia southward are lacking). 

 This of course, was accomplished by the same means as in the 

 other members of the Atlantic fauna, and it is not astonishing since 

 this species is not only upon the Piedmont Plateau, but also on the 

 Coastal Plain. 23 



Further details cannot be given, and chiefly it is impossible to fix 

 the geological time when Strophitus crossed the mountains. As 

 has been said, possibly this happened repeatedly, presumably in the 

 Tertiary, and may have happened even later. 24 More information 

 as to its southern range may furnish additional evidence, and con- 

 firm the view that the crossing of the divide was effected in the 

 northern section of the Alleghenies, and not in the south. At pres- 



23 1 found it in Delaware River, Penns Manor, Bucks Co., Pa. Its dis- 

 tribution upon the Coastal Plain is yet incompletely known, but it seems to 

 be represented there at least by a local (or ecological?) form, Strophitus un- 

 dulatus. 



21 At present, this species has a continuous range from West to East in 

 the state of New York, and this, of course, belongs to the Postglacial time. 



