FRESHWATER FISH FAUNA 207 



dominant in the saline, alkaline, and frequently warm waters of the 

 Death Valley system (Miller, 1948). Two of the four genera are well 

 isolated relicts, Crenichthys, Fig. 16 (Hubbs, 1932), and Empe- 

 trichthys, each with two species, and the two remaining genera 

 (Fundulus and Cyprinodon) have their closest relatives in southern 

 United States and northern Mexico. The importance of this sec- 

 ondary family in indicating past connections of such disrupted 

 drainages as the Death Valley system has been substantiated by 

 geological evidence. 



CENTERS OF ENDEMISM 



In correlation with the physiographic disruption of the West 

 during late Tertiary and Quaternary times, the fish fauna has 

 differentiated within a group of isolated basins each with a more or 

 less high incidence of endemism and generally having few strictly 

 freshwater species in common (Hubbs and Miller, 1948). Seven 

 main centers of endemism may be recognized constituting the fol- 

 lowing drainage systems: (1) Colorado, (2) Sacramento, (3) 

 Klamath, (4) Columbia, (5) Bonneville, (6) Lahontan, and (7) 

 Death Valley (Tables I-II; Fig. 1). Not all the fishes inhabiting the 

 West are included in these 7 systems since there are certain inde- 

 pendent basins (for example, between the Lahontan and Columbia, 

 Lahontan and Bonneville, and Lahontan and Colorado systems) 

 that harbor a few primary species (about 13 in all) unknown else- 

 where. The faunas of these extralimital systems are discussed later. 



Most species that occur in more than one of the seven isolated 

 drainages belong either to the semi-marine groups (e.g., lampreys, 

 sturgeons, smelts, most salmonids, and sticklebacks) or are moun- 

 tain-creek types (such as mountain whitefish, cutthroat trout, and 

 certain suckers and minnows, particularly the ubiquitous speckled 

 dace, Rhinichthys osculus). The montane types probably attained 

 their widespread distribution by means of stream captures, head- 

 water distributary connections (like Two-Ocean Pass, Wyoming; 

 Evermann, 1892, pp. 24-28, PI. II), or through stream shifting across 

 low divides. The distribution of the genera of primary fishes that 

 are common to a number of the basins (such as the suckers, Cato- 

 stomus and Pmitosteus, and the minnows, Gila, Ptychocheilus, 

 Rhinichthys, and Siphateles) probably took place largely in Pliocene 

 or early Pleistocene times. 



