xlviii FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. V. 



The presence in the Rio Yaqui of so many fishes from the Rio 

 Grande basin can be thus interpreted: The head waters of the Rio 

 Paphigochic, a tributary of the Rio Yaqui, lie east of the central range 

 of the Sierra Madre. That portion of this stream no doubt formerly 

 had its outlet into the Rio Conchos and in this way became stocked 

 with fishes from the Rio Grande. The fact that the fauna of the Rio 

 Yaqui is so much like that of the isolated river basins in northern 

 Mexico rather strengthens this belief, though its ultimate proof 

 must depend on the geologist. The same is probably true concerning 

 that portion of the Rio Mezquital which lies east of the central range 

 of the Sierra Madre. It would be interesting to compare the fauna 

 of the upper with that of the lower portion of these rivers. No col- 

 lection of fishes has been made in the head waters of the Pacific Coast 

 streams between the Rio Yaqui and the Rio Mezquital, but we may 

 reasonably suppose that the portions of those streams east of the main 

 range of the Sierra Madre contain Rio Grande fishes. From the Rio 

 Presidio, which flows into the Pacific Ocean near Mazatlan, a col- 

 lection of fishes was made (at Presidio) a short distance from its 

 mouth by Dr. D. S. Jordan. Of the twelve species taken here, not 

 one belongs to the northern fresh-water fauna. Two species* are 

 brackish water fishes, onef belongs to the tropical fresh-water fauna, 

 while the other species J are salt-water forms which have become more 

 or less established in fresh water. The fishes listed from the lower por- 

 tion of the Rio Presidio are much the same as those from the Rio San 

 Jose in lower California. It is quite probable that the list of fishes 

 from these two places fairly well represent the fish fauna of the lower 

 portion of all of the Pacific Coast streams between the Rio Yaqui and 

 the Rio Grande de Santiago. As mentioned before, it is probable that 

 the portions of the upper tributaries of these rivers which lie east of the 

 main range of the Sierra Madre have in them fishes belonging to the 

 Rio Grande fauna, and this fauna has been obtained by capturing 

 upper tributaries of the Rio Grande. All of these Pacific slope 

 streams reach the sea evidently in a long series of cascades and falls 

 which are sufficient barriers to prevent fishes from migrating in either 

 direction. Fishes are limited in their ability to ascend falls, and we 

 have reason to believe, as stated below, that they are unable to go 

 over any considerable falls and become established below them. The 

 Falls of the Yellowstone is the dividing line between the fauna of the 

 Missouri River and that of the Upper Yellowstone. The Shoshone 



*Paecilia butleri Jordan, Pcecilia presidionis Jordan & Culver. 

 \Cichlasoma beani (Jordan). 



lAgonostomus monticola (Bancroft), Philypnus dormitor (Lac6pede), and 

 Chonophorus taiasica (Liechtenstein) are found only in fresh or brackish water. 



