336 THE PEARLY FBESH-WATEB MUSSELS— SIMPSON. vol. xvm. 



Oriental region, without being convinced that the TJnione fauna of 

 that area is somewhat closely related to that of the Laramie beds and 

 the Mississippi Valley, and the conclusion seems reasonable that a 

 migration took place, perhaps during or shortly after the Laramie 

 epoch, over an old, now submerged, landway, either from Asia to 

 North America or vice versa. It is, I believe, more probable that this 

 fauna developed in the western continent than the eastern, for, as we 

 have seen, a few prophetic types of it appeared in the Xorth American 

 Jurassic, while the earliest recorded existence in the Old World of 

 species wliich seem intimately related to it is in the later Cretaceous or 

 earlier Tertiary, While some eight or ten groups of Unios and Ano- 

 dontas now living in the Oriental region bear such a strong resem- 

 blance to simihir assemblages in the United States that at first sight 

 they seem to be the same, I believe every one of them to be distinct, 

 and it seems probable, when it is taken into consideration how slowly 

 the Naiades change, and the fact that the forms of the Laramie groups 

 have scarcely altered specifically in our own country, that if any such 

 migration and separation took place, it occurred a long time ago. 



It is quite likely that about this time members of some of the Lara- 

 mie groups found their way into Mexico, Central America and Cuba. 

 It is very probable that this area was separated from South America 

 at that time, and for a considerable period since, as no interchange of 

 Naiades is known to have taken place between the continents until per- 

 haps during the Pliocene, or at least since the last union of land areas 

 took place. No North American form is found in South America, and 

 the few Glaharis and the Mycetopoda that have entered the Central 

 American province from the south, have scarcely changed specifically. 

 This Laramie Unio fauna in Mexico and Central America has every 

 appearance of having been in some way isolated from the rest of 

 North America, as if it had developed under insular conditions. 

 Almost all of the older groups of the Central American region have 

 their analogues in the Mississippi Valley to-day, yet very few species 

 of these Mexican groups come north of the Rio Grande Kiver; and 

 while there is a slight mingling of forms of the two provinces, yet the 

 groups can be separated, and the southern Naiad fauna has a distinct- 

 ive appearance, notably in the much softer, more silvery, nacre, and an 

 indefinable difference in the epidermis. I should say that these older 

 Central American fauna groups bore about the same relation to those 

 of the Mississippi Valley as do many of those of the oriental region. 

 Judging from the apparent evidence of the Naiades, one would suppose 

 that after the migration of these old forms into Mexico and Central 

 America, they were isolated from the rest of North America long 

 enough to take on certain peculiarities, and that after this the two 

 areas were connected again, and that since the connection a few species 

 of Unios and Anodontas of the present Mississippi Valley groups had 

 migrated southward. I am aware that what is known of the geology 



