42 THE NAUTILUS. 



Valley in Cretaceous times, and would probably seem to have been in 

 later Cretaceous or early Tertiary times. 



It would seem most probable that the primitive ancestral form of 

 the complanatus group also reached the Atlantic region by the same 

 northern route. Although the greatest diversity of forms belonging 

 to it J3 now peculiar to the southeastern Atlantic states and, under 

 the axiom already quoted, would seem to indicate that that region 

 was the center of distribution of the group, the weight of evidence 

 is against it. 



As has already been stated, the invasion of the sea up the Missis- 

 sippi Valley in Cretaceous times prevented any emigration towards 

 the east from the southwest during nearly the whole of the Tertiary 

 Period. Moreover, during the greater part of that time this region 

 itself was covered by the sea. The invasion of the southeastern 

 States by the present Unione fauna must necessarily have been, 

 comparatively speaking, a recent one. Had it been coincident with 

 the advance of the southwestern fauna that now occupies the Gulf 

 States to the west of the Alabama River, there would, no doubt, be 

 some evidence left in the present fauna of those States. But there is 

 none. The present distribution of the group shows that it stops 

 abruptly before reaching the Alabama River. With one exception, 

 the group is not represented in the fauna of the Alabama at the 

 present time, nor is it found at all in any of the Gulf States west of 

 that river. 



The exuberant variability of the group in the southeastern States 

 at the present time would also seem to indicate that it is compara- 

 tively a new comer, and that it has not even yet had time to settle 

 down to stable lines of development. 



On the other hand, the homogeneity of the group as a whole, in 

 spite of its extreme variability within the group limits, would indi- 

 cate that it is of ancient origin. The extraordinary range of the 

 typical species, from Lake Superior to the Atlantic and south to 

 Georgia, is evidence in the same direction. 



Taking everything into consideration it must be said that, with 

 the exception of a comparatively small number of species that from 

 one cause or another have been able to get into the South Atlantic 

 States from the faunas of the Alabama and Tennessee systems, all 

 the evidence goes to show that the characteristic fauna of that region 

 has been derived from the north. 



