INTEODUCTION. xxvu 



Eastern Asia, whilst to the last-named family has been referred a marine Indo-Pacific 

 genus, with fresh-water representatives in Australia. 



The Nearctic Eegion may be divided into three sub-regions, the first corresponding 

 to the United States west of the Eccky Mountains and Northern Mexico to the west 

 of the Sierra Madre. This, the Californian sub-region, is especially remarkable for the 

 pau.city of true fresh-water fishes. Esocidse and Percidae, in addition to the endemic 

 nearctic families, are entirely absent, whilst the Siluridte and Centrarchidse are each 

 represented by only a single species. The Cyprinidse are fairly abundant, several 

 peculiar generic types inhabiting this sub-region. 



The fish-fauna of the rivers of Sonora and Sinaloa is very little known ; the few 

 fishes which have been recorded from the Pio Sonora show that its fauna is similar to 

 that of the Rio Colorado, and the same may be said of that part of the Rio Yaqui 

 W'hich drains the western slope of the Sierra Madre. Only two strictly fresh-water 

 fishes are known from streams of Sinaloa and Jalisco to the west of the Sierra Madre ; 

 one of these [Ciclilosoma heani) is a neotropical type, the other [Moxostoma mascoUe) 

 a nearctic one. 



The second sub-region of the Nearctic Region includes Alaska, Canada, the United 

 States east of the Rocky Mountains, and Mexico east of the Sierra Madre and north 

 of the Lerma System. This sub-region may be characterized in nearly the same terms 

 as those used to define the Nearctic Region as a whole : there is a northern zone 

 in which the Salmonidte are dominant ; further south the Cyprinidge become well 

 established, and in the Mississippi System attain their maximum, together with the 

 Cyprinodontinse, Centrarchidse, and Etheostomatinse ; in the Rio Grande a number of 

 characteristic types disappear. These changes in the character of the fish-fauna render 

 it necessary to divide this sub-region into several provinces, the southernmost of which 

 (Rio Grande Province) comprises the Rio Grande System, with the rivers of the 

 Mexican plateau north of the Lerma System and the streams of the Atlantic slope in 

 Tamaulipas and Northern Vera Cruz. 



A number of streams in Chihuahua and Durango flow in the direction of the Rio 

 Grande, but never reach it, terminating in lakes which vary in magnitude according to 

 the season. As has been pointed out by Dr. Meek, the nature of the fish-fauna justifies 

 the supposition that these rivers were formerly tributaries of the Rio Grande. For a 

 similar reason he considers that the portions of the Yaqui and Mezquital Rivers to the 

 east of the Sierra Madre also at one time belonged to the Rio Grande System, and that 



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