34 THE NAUTILUS. 



If these theories of the ancient pre-glacial drainage of this region 

 are correct, it does not require much imagination to see how, from 

 their ancient place of origin in the west, the primitive ancestral 

 forms of our present fauna were enabled to spread to the east up 

 these ancient waterways to the headwaters of these pre-glacial 

 streams, and then, during the many elevations and sinkings of 

 Tertiary times, through the stream transference consequent upon 

 such orographic changes, this immigration of the primitive fauna 

 was transferred into the eastern drainage, and thus became the an- 

 cestral stock of the present Atlantic fauna. 



That this emigration from the west to the east was a very early 

 one, there can be no doubt, and that it was long antecedent to the 

 Glacial Period seems beyond question, both from a geological and a 

 zoological standpoint. The fact that not only from the Glacial 

 Period, but for long ages prior to that time, the Appalachian system 

 must have been a barrier to the entrance of the western fauna into 

 eastern waters would seem to be beyond question, and this view is 

 strengthened and corroborated by the fact that the two faunas have 

 been so long separated that they have become specifically differ- 

 entiated in the great majority of cases. The time that is involved 

 in such a change must be very great. That it must be so is shown 

 by the fact that the fossil Unios found in the inter-glacial drift of 

 eastern Canada are the same as the recent examples of the same 

 species found to-day. It is probable that this emigration from the 

 west took place after the primitive fauna of early times had begun 

 to mutate under the peculiar influence of the later Cretacic times, 

 and while, of course, there is much that is indefinite and purely 

 speculative in regard to these questions, there are some facts, which 

 seem to point with some directness, as to when that migration might 

 have taken place. 



One of the characteristic species of the Atlantic fauna is Lamp- 

 silis radiata, which extends at the present time along nearly the 

 whole extent of the Atlantic drainage. It is very closely related to 

 another characteristic species of the Mississippian fauna, the Lamp- 

 silis luteola, and, indeed, these two specimens are so closely related 

 that while in the main there is no difficulty for the average student 

 to separate them, yet oftentimes there are individual specimens 

 which are very difficult to place with entire satisfaction. 



( To be continued.} 



