No. 443] FRESH-WATER FISHES OF MEXICO. 783 



crowded its way over the divide and has become even more firmly 

 established in the Pacific coast streams of Sonora than has the 

 Rocky Mountain fauna. 



The South and Central American faunas prevail largely as far 

 north as the City of Mexico. The few forms which extend far- 

 ther north apparently keep to the lowland streams ; especially 

 is this true on the Pacific side. The most northern representa- 

 tive of the South American fauna, one of the Cichlids, 1 is found 

 in Mazatlan. On the east coast this family has a representative 

 in Texas. The fauna of the Lerma, the only river basin exten- 

 sively studied, is quite distinct from either North or Central and 

 South America. This fauna is richer and more characteristic 

 than was formerly supposed. 



Mexico in general is not a well watered country. Nearly all 

 of the small streams and many of the large ones become much 

 reduced in size by the end of the long dry season, and such 



the Mexican plateau the largest and most important lakes are 

 found in the Lerma basin; Lake Chapala, the largest and the 

 only one which has a large river for outlet and inlet, sustains 

 the largest fish fauna. Patzcuaro, a large lake with no inlet or 



number of individuals. In view of the fact that more species of 

 fishes belong to tropical Mexico than to a like area farther north 



wholly within the tropics should contain so few species. This 



