384 The American Naturalist [S fay, 



It is possible that here we have an opportunity to make 

 some kind of an estimate as to the time required in develop- 

 ing species and varieties among the Unionidse. It is well 

 known that the Laramie strata of the northwest, belonging 

 perhaps to the upper cretaceous or earlier Tertiary systems 

 contain the remains of a large number of Unios which 

 appear to be very closely related to existing Mississippi Val- 

 ley forms, and are probably their progenitors. Some of these 

 old fossils are so much like certain recent species that they 

 might easily be taken for them by an expert, and nearly or 

 quite all of them can be placed in existing groups. 



Yet it is more than probable that the great variety of 

 changes that have been produced in the Mississippi Valley 

 forms which now inhabit the St. Lawrence drainage area have 

 taken place since the Ice Age began to draw to a close, 

 because it is almost certain that all fluviatile and lacus- 

 tine life under the ice sheet was destroyed, and that any 

 forms closely allied to those of the Mississippi Valley now 

 found north of the Height of Land migrated there since. It 

 is held by most glaciologists, I believe, that the Glacial Epoch 

 reached down probably to within from 10,000 to 20,000 years 

 of the present. This amount of time might probably be 

 taken as the age of these peculiar forms of St. Lawrence 

 Mississippi Naiades. 



Unio radiatus, ochracewt, cariosus, heterodon, tappanianus, and 

 Margaritana undulata, which are found in the Atlantic drain- 

 age south of the line of the ice cap, and which are all closely 

 related to common Mississippi Valley forms are probably 

 older, and may have been derived from some migration made 

 from the western to the eastern drainage at a much earlier 

 date. At any rate I believe that all the Uniones which belong 

 properly in the Atlantic drainage system were derived at one 

 time and another from Mississippi Valley species ; that some 

 peculiarity of environment common to this entire region 

 has had a tendency to dwarf them, to simplify their forms and 

 dull their colors. 



