Oumings — Morphogenesis of Platystrophia. 125 



indicates, as pointed out by Ulrich and Schuchert,* a connec- 

 tion of this latter area with Europe during the early Ordovi- 

 cian, and its separation from the Mississippi area. I believe 

 also that the strong similarity of P. hiforata of the Champlain 

 trough to the European lower Ordovician forms indicates close 

 contemporaneity of the two terranes. 



Again, the Clinton Platystrophia of this country is almost 

 identical with the European P. hiforata, and therefore indi- 

 cates a migration of European forms into American waters of 

 the Mississippi area at the close of the Ordovician. This also 

 coincides with the views of the authors just mentioned. The 

 Niagara species, P. dentata. is identical with the Gotland 

 form, and differs from the Clinton form, thus indicating a 

 second migration of European forms during Niagara time. 

 The bearing of this evidence upon Dr. Weller's views has been 

 pointed out in another place. 



A point perhaps worth calling attention to, is the erroneous 

 ideas current in regard to the distances to be covered by these 

 migrating forms and the routes pursued, that we obtain from 

 Mercator projections and globular and stereographic projec- 

 tions on any plane but that of the Equator. By reference to 

 the map, fig. 24, it is clear that the route of a form migrating 

 from Gotland, for example, into the Mississippi Valley, is no 

 more circuitous by way of Hudson Bay than by way of the 

 Atlantic. It is even less so. 



Laws of the Evolution of Platystrophia. — It is not to be 

 assumed that the history of Platystrophia outlined in the pre- 

 ceding pages is in any way complete, or that it could ever be 

 made complete by any amount of research. Every worker in 

 this department of science is painfully aware, when he appeals 

 to the original sources of information, the rock-strata, of the 

 many volumes of the record irretrievably lost, and of the many 

 events never recorded at all. My aim has been to have before 

 me specimens of all the forms of Platystrophia known from 

 American terranes ; and this has been rendered possible 

 through the kindness of the gentlemen mentioned above, to 

 whom I once more express my gratitude. I should have been 

 glad, indeed, to have had the same opportunity of exam- 

 ining all the European forms; but for the present this is 

 impracticable. Imperfect access to available material is, there- 

 fore, added to the imperfections of the original sources of 

 information. What, therefore, rests on theory the reviewer 

 must accept or reject according to his convictions of its merits 

 or demerits. The facts, many of which I have had the pleas- 

 ure of ascertaining and many of which are due to the labors of 

 others, I have endeavored to present in such a way that the 

 risk of confusing them with theory will be avoided. 

 * Bull. N. Y. State Mus., No. 52, 1902, pp. 633-663. 



