194 R. R. MILLER 



Nevada, Leuciscus turneri (Lucas, 1900), is likely identical with a 

 modern genus (Gila); the age of this formation, first thought to be 

 Miocene, was discussed by Stirton (1936, 1939). 



A late Pliocene to early Pleistocene fauna from southern Idaho 

 and eastern Oregon contains representatives of 6 families, including 

 a catfish of the genus Ictaliiriis and a sunfish likely of the genus 

 Lepomis ; the latter genus is now restricted to eastern North America 

 and Ictaluriis is not native on the Pacific slope north of the Yaqui 

 basin (Figs. 5 and 9). Both genera represent invaders from the East 

 whose extinction in the Snake River basin in early Pleistocene times 

 is probably correlated with lowering of water temperatures below 

 the minimum spawning requirements of these fishes. Most of the 

 fossils representing this fauna were discussed by Cope (1883, pp. 

 153-165). 



This brief summary of the sketchy fossil record indicates the ur- 

 gent need for research on the paleoichthyology of the later Cenozoic, 

 particularly for comparisons of the fossils with their living relatives. 

 Much of the material is not identifiable because of our inadequate 

 knowledge of the osteology of modern fishes. 



PRIMARY AND SECONDARY GROUPS 



The primary freshwater fish fauna of North America (Fig. 1) 

 comprises 21 families with approximately 600 species. Although these 

 are mapped by areas that generally follow major watersheds, it is 

 not intended that these areas portray zoogeographic regions. The 

 numbers, representing families, genera, and species, become less 

 accurate south of the United States border. The richness of the 

 Mississippi Valley fauna is noteworthy, as is the absence of primary 

 fishes in the West Indies. In general, there is a trend toward more 

 species to the south, perhaps correlated with more equable water 

 temperatures and a greater diversity of habitats. The apparent 

 richness of the Arctic fauna is largely the result of invasion of the 

 Red River by Mississippi fishes during late Pleistocene times. 

 Similarly, as a result of Glacial and Postglacial connections, the 

 fauna of the Great Lakes basin is strikingly similar to, though 

 smaller than, that of the Mississippi Valley. 



The Plateau Region of northern Mexico has provided a broad 

 highway over which many Nearctic types have traveled southward. 

 Although the bulk of these fishes are stopped by the east-west chain 



