Occasional Papers of the Museum of Zoology 7 



j-resent fauna. Through the migration which they were able 

 to make up these ancient rivers, these early primitive forms 

 were enabled to reach and establish themselves in the head- 

 Avaters in the Laurentian Highlands and in the ancient Ap- 

 palachian System.* 



Isolation. — The first, and perhaps the greatest, factor of 

 isolation which has resulted in the development of our recent 

 fauna, occurred in early Cretaceous times, w^hen by the sink- 

 ing of a large section of the earth's crust in the Gulf region, 

 what is known as the Mississippi Embayment was formed. 

 This affected a triangular piece of territory extending along 

 the line of the Gulf of Mexico from eastern Texas to the 

 middle of Alabama, and thence northerly to a point above the 

 present junction of the Ohio and Mississippi Rivers. The 

 whole land between these two lines sank at least 5,000 feet, 

 and the sea was admitted as far north as Memphis. This 

 continued entirely through Cretaceous and Tertiary times until 

 by the gradual uplifting of the land and the deposits of sedi- 

 ment brought down by the rivers on either side the depression 

 \\d.s filled up and the present systems of drainage were estab- 

 lished. So far as the Unionidae are concerned, the important 

 result of this fact was that during all this time the Unios 

 which remained in the rivers west of the embayment were 

 entirely separated from those, which, prior to the great land- 

 slide had been able to affect a lodgment in the headwaters 

 of the ancient rivers flowing from the Cumberland Plateau. 

 This created two centers of development and distribution; the 

 one west of the Mississippi, and the other in eastern Ten- 

 nessee. 



* There is a larg-; amount of evidence tending to show that there was con- 

 siderable migration from the west of the Mississippi, northeastward into the 

 Laurentian region after the Mississippi Embayment, during Tertiary times, but 

 ■"that is another story" and does not in any way conflict witli the general state- 

 ments herein made. 



