FRESHWATER FISH FAUNA 217 



fossil minnows and suckers, along with careful comparison of the 

 living fishes, should help to determine whether the fauna entered 

 this region by more than one route. 



One other well-isolated basin shows a curious faunal mixture. 

 Railroad Valley, in eastern Nevada, was the site of a large Pluvial 

 lake (Hubbs and Miller, 1948, p. 90, Map 1, No. 60). The sur\dving 

 waters contain chubs referable to Siphateles bicolor and an endemic 

 cyprinodontid, Crenichthys nevadae (Fig. 16). The only other species 

 of Crenichthys, C. baileyi, is known from the remnants of the adja- 

 cent Pluvial White River, once a permanent tributary of the Colo- 

 rado River. Structural troughs lead from Railroad Valley in a 

 southeasterly direction toward the Colorado River suggesting, along 

 with the mutual occurrence of Crenichthys, that Railroad Valley 

 once drained in that direction, perhaps in early Pleistocene time. 

 Siphateles is particularly characteristic of the Lahontan basin and 

 presumably entered Railroad Valley from the north after the con- 

 nection between that valley and the Colorado system was severed. 



SPECIES CROSSOVERS BETWEEN EASTERN AND WESTERN 



NORTH AMERICA 



A study of the extralimital ranges of western and eastern fishes 

 shows that 29 species have taken part in recent crossings of the Con- 

 tinental Divide. Possibly some of these transgressions are the result 

 of human intervention (Lindsey, 1956, pp. 780, 782). These fishes 

 comprise 23 genera in 9 families, 3 of which are primary. About 

 twice as many species have moved from east to west as from west to 

 east; where the crossing was effected by stream capture, this sug- 

 gests that the western rivers have been the more active in the 

 piracy. A total of 19 eastern species has entered the Columbia 

 complex of streams and the Yaqui drainage, whereas only 10 western 

 species have invaded eastern waters at points from British Columbia 

 to Wyoming. 



Stream capture by the Yaqui River of a drainage that once was 

 connected with the Rio Grande has given the Yaqui 8 eastern species 

 (7 primary, 1 secondary) ; except for a Gila and Ictalurus pricei, none 

 of these is recorded elsewhere on the Pacific slope. The eastern ele- 

 ment makes up 26 per cent of the total fauna and 58 per cent of the 

 primary fishes inhabiting the Yaqui. A well-established route of 

 two-way faunal exchange has taken place between the Snake River 



