340 THF PEAHLY FFESR-TTJTEE MUSSELS— SIMPSON. voi..xvni. 



their relationship to North American forms, I can not do better than 

 to quote from Dr. C. A. White :^ 



It lias already been shown that the living Uuionidae of all Europe depart compara- 

 tively little from the primary, typical, oval form, and smooth or plain surface. These 

 are the characteristics, so far as I am aware, of all the fossil species, save one, that 

 are found in the strata of Avestern Europe, including those from the Wealden and 

 Cretaceous rocks. The exception referred to is Vn\o toulousanii, Matheron, from the 

 Lignite strata of the department of the mouths of the Rhone, which, while differing 

 but little in form from the other fossil and living Unionida- of western Europe, is 

 marked by small plications upon its postero-dorsal surface. In Slavonia, Croatia, 

 Dalmatia, and other parts of southeastern Europe, however, the fossil Tertiary spe- 

 cies of Unio are much more numerous than the living species of the family are in the 

 whole continent. Furthermore a large proportion of the types of these fossil species 

 of southeastern Europe are as distinctively " North American " in character as those 

 are which now live in the Mississippi Eiver and its tributaries. 



From these facts the inference seems to be a natural one that the living UnionidaB 

 of all Europe are descended from those which are represented by the Mesozoic and 

 Cenozoic fossil species of the western part of that continent ; while the line of descent 

 of the fossil sjiecies of southeastern Europe has evidently been cut off by disastrous 

 changes of the physical conditions necessary for its perpetuity. The fact that these 

 last-mentioned fossil species are identical in type with those of North America jire- 

 sumably indicates, though it does not necessarily prove, a community of origin ; in 

 which case they must have reached their present separated regions by some ancient 

 continental connection now destroyed. 



Among the Pliocene Unios from Slavonia there are many which almost 

 absolutely agree with species living in the United States, belonging to 

 the groups of ^^^ clavus, TJ. trigonus, U. perplewus, U. pustulosus,and other 

 well-known Mississippi Valley assemblages; and U. sibinensis, Tenecke, 

 is almost exactly like U. houstonensis, Lea, of Texas; U. neitmat/ri^ 

 Tenecke, is the counterpart of U. modicus. Lea, of Alabama ; U. stolitzl-ai, 

 Neumayr, is a nearlj' perfect reproduction of U. cesopuSj Green, from the 

 Ohio River, and U. novsl-alensis, Tenecke, is like a slightly roughened 

 U. pyramidatus. Lea, from the same stream. Other species from the 

 Pliocene beds of Slavonia almost as closely resemble IJ. leai, Gray, and. 

 17. oshecM, Lea, of China. 



It seems not unreasonable, no matter where these striking types of 

 Unios and Anodoutas may have originated, whether in North America 

 or the Old World, that they afterwards spread so that they occupied 

 the greater part of Asia, Europe, except its western part, and possibly 

 Africa, whose Unione fauna is, by the cliaracters of the shells, apparently 

 closely related to the Tertiary fauna of Europe, and that of India at tlie 

 present time. It may be that the extreme cold of the glaciers exter- 

 minated or drove these forms to the region south of tlie Himalayas in 

 Asia, and that the simple and probably more hardy species of western 

 Europe spread rapidly to the eastward and southward after the Glacial 

 epoch until they peopled the vast Palearctic region. But it seems 

 probable that the European and northern Asiatic Anodontas, whose 

 descendants now inhabit North America west of the Rocky Mountains, 

 crossed over during the late Tertiary, as some of the forms how found 



'Bull. U. S. Geol. Surv., Ill, Art. 23, p. 621, 1877. 



