THE NAUTILUS. 41 



broke through and separated the ancient Cumberland Plateau, which 

 prior to that time extended continuously from the eastern mountains 

 into western Texas. It admitted the sea to a point, as above stated, 

 north of the present junction of the Ohio and Mississippi, and during 

 nearly the whole of Tertiary times there was a body of salt water 

 between the western highlands and the eastern portion of the Cum- 

 berland Plateau, in what is now Tennessee and Kentucky. This 

 invasion of the sea was, of course, an absolute barrier to any com- 

 munication between the Unione faunas of the two regions. 



The evidence afforded by the present distribution of the species of 

 the group, to which these species belong, shows that its center of dis- 

 tribution, as affecting the present fauna, was in the southwest. Not 

 only is the southwest the region of the greatest variation in the spe- 

 cies of this group, but, while it extends from Texas easterly along 

 the Gulf States as far as Alabama, and even into Georgia and 

 Florida, and extends north through the entire Mississippi Valley to 

 the Appalachians on the east and the Arctic regions on the north, 

 there is no representative of that group found to-day, so far aa 

 records show, in any part of the Tennessee Valley. The inference 

 to be drawn from this fact is that the group originated in the west, 

 and after the great landslide of Cretaceous times. Another ex- 

 ample, bearing upon the same general fact, is the distribution of the 

 group, of which the well-known Quadrula rubiginosa is a leading 

 example. If we are to rely upon the proposition that the center of 

 distribution is the region where there is the greatest abundance of 

 individuals and of specific forms, it would seem certain that this 

 group originated in the southwest and from thence spread eastward 

 to its present distribution. But Quadrula rubiginosa, like Lampsili* 

 luteola, is not found in the Tennessee Valley. Its distribution 

 through the Gulf States is similar to that of the Lampsilis, and its 

 distribution north through the Mississippi and Ohio valleys is 

 exactly the same. Like luteola, it is found in the Lake Erie, but 

 for some reason, that we do not now know, no form of that group 

 ever succeeded in obtaining a foothold at any time in the northern 

 Atlantic fauna. 



If the inferences to be drawn from these facts and others like 

 them are to be relied upon, there would seem to be good reason to 

 infer that the emigration, which was the beginning of the Atlantic 

 fauna, took place after the invasion of the sea in the Mississippi 



