WHITE ON PALEONTOLOGY. 621 



It bas aliealy biea shown that the liviug Uiiionidcc of all Europe 

 depart comparatively little from the primary typical oval form and 

 smooth or plain surface. These are also the characteristics, so far as I 

 am aware, of all the fossil species, except one, that are found in the 

 strata of Western Earope, including those from the Wealden and Cre- 

 taceous rocks. The exception referred to is U)iio toulouzanii Matheron, 

 from the Lignitic strata of the department of the Mouths of the Ehone, 

 which, while differing but little in form from the other fossil and living 

 Unionidce 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 species of 

 Unio are much more numerous than the fossil species are in Western 

 Europe, and also more numerous than the living species of the family 

 are in the whole continent. Furthermore, a large proportion of the types 

 of those fossil species of Southeastern Europe are as distinctively " North 

 American" in character as those are which now live in the Mississippi 

 River and its tributaries. 



From these facts, the inference seems to be a natural one that the liv- 

 ing Unionidce of all Europe are descended from those which are repre- 

 sented by the Mesozoic and Cenozoic fossil species of the western part 

 of that continent; while the line of descent of the fossil species of South- 

 eastern Europe has evidently been cut off by disastrous changes of the 

 physical conditions necessary to its perpetuity. The fact that these last - 

 mentioned fossil species are identical in type with those of North Amer- 

 ica, presumably indicates, although it does not necessarily prove, a com- 

 munity of origin ; in which case they must have reached their present 

 separated regions by some ancient continental connection now destroyed. 

 If this was not the case, it seems necessary to conclude that these geo- 

 graphical groups of identical types were produced in separate lines of 

 descent, the whole evolutional history of each having been confined to 

 its own hemisphere; the European line ending before the close of the 

 Tertiary period, and the American line continuing in fiill force, reaching 

 its culmination in the fluviatile waters of the present period. 



Up to the present time, there have been twenty-four species of the 

 Unionidce discovered in the strata of Western North America, ranging 

 from the Jurassic period to the earlier Tertiary, and without doubt 

 many more species remain to be discovered there. 



All except two of these species have the well-defined characteristics 

 of true Unio, as that genus is defioed and understood by the writer. 

 One of these species is Anodonta propatoris White, from the Judith liiver 

 group (PostCreteceous), and the other is Mcirgaritana nehrascensis Meek, 

 from the Dakota group (earlier Cretaceous). Less than one-half of 

 these species of Unio have either the oval, elongate-oval, or arcuate 

 outline and plain surface, while the others are of various types, a large 

 proportion of them being peculiarly North American, with a tendency 

 to excessive shortening of the shell in front of the beaks, in some cases 



