1895. 



PROCEEDINGS OF THE NATIONAL MUSEUM. 343 



TheUnios undoubtedly date back well into the Jurassic; probably 

 into the Triassic. The post-Cretaceous Unio fauna of the Northwestern 

 States is evidently closely related to the fauna of the Mississippi Val- 

 ley, and this seems to be related to that of Mexico, to tlie oriental 

 fauna, and more distantly to that of tropical Africa, as well as to the 

 Tertiary forms of eastern Europe and Siberia. The Unios of Australia 

 and South America are apparently closely related to those of the 

 Australian region. There seems to be, too, a general relationship 

 between the Mutelidic of Africa and South America. These IMutelids 

 and the Unios which bear the embryos in the inner gills have perhaps 

 formerly occupied extensive areas in the northern hemisphere, and 

 may have been supplanted by more modern forms. 



