Evermann — A New Trout from Lower California. 23 



The fourth locality in Mexico from which trout have heen 

 reported is the headwaters of the Rio Yaqui. The exact 

 locality is not known; it may be in northeastern Sonora or in 

 northwestern Chihuahua. The occurrence of trout in this region 

 is mentioned by Dr. Meek,* on the authority of Mr. John 

 Ramsey, General Manager of the R. G., S. M. & P. R. R., who 

 says " that a trout is quite abundant in the upper tributaries 

 of the Rio Yaqui . ' ' 



Dr. Meek inadvertently records this trout as a member of 

 the Colorado River fauna under the name Salmo irideus, which, 

 of course, is quite erroneous, as the trout of that river is not a 

 rainbow trout, but one of the cutthroat series, Salmo pleuriticus. 

 There is considerable geologic evidence that the portions of the 

 western rivers in this region which are east of the Sierra Madre 

 Mountains were formerly the upper tributaries of streams flow- 

 ing eastward. Some of the streams flowing to the Gulf of Cali- 

 fornia have cut their way back, thus capturing the headwaters 

 of the eastern streams and with them their portion of the eastern 



fish-fauna, t 



This explanation would account for trout in the headwaters 

 of the Rio Yaqui, but would give them a Rio Grande origin 

 which would make them Salmo spilurus or a derivative from 

 that species. This may also be the origin of the trout which 

 occur in the streams west of Durango. But it is wholly im- 

 probable that trout could have reached western drainage in 

 Lower California from the Rio Grande; the origin of the San 

 Pedro Martir trout must be explained in some other way. 



Trout are known to occur in Arizona in a number of moun- 

 tain streams all of which, however, are tributary to the Colorado. 

 Between these streams and Lower California lies the broad semi- 

 desert region of northern Sonora, the Gulf of California, and 

 the eastern portion of the peninsula, a desert region on the west 

 of which are impassable mountains, beyond which lies the 

 stream in which the trout are found. Trout might have come 

 down the Colorado, but they would have met these same im- 

 passable barriers. And westward from the lower course of the 

 Colorado is the broad expanse of desert across which trout can 

 not now possibly pass. What may have been the conditions 

 long ago can not, of course, be definitely known, but it is con- 



* See Meek, op. cit. p. XXVII. i Meek, op. fit. p. 96. 



