xxvi FIELD COLUMBIAN MUSEUM ZOOLOGY, VOL. V. 



The Valley of Mexico is an independent drainage basin, though judg- 

 ing from the nature of its fish fauna, .at some previous time its waters 

 probably flowed into the Rio Lerma. The northern limit of the 

 basin of the Rio Lerma is at Zacatecas. 



A glance at any map of Mexico reveals the fact that the largest 

 western rivers north of the Rio Grande de Santiago rise to the east of 

 the central range of the Sierra Madre Mountains, or in the western 

 portion of the Central plateau. The western Sierra is exceedingly 

 rough and its western slope very steep. To such an extent is this 

 the case that of the seven railroads which have western terminals 

 not one has yetlreached the Pacific coast. 



The rainfall in Mexico is variable, the greatest precipitation being 

 in the State of Vera Cruz. On the Isthmus of Tehuantepec the rains 

 begin about the first of May, and continue into December, though 

 there are occasional rains until about the first of February. In the 

 neighborhood of the City of Mexico and the Lerma Valley the rains 

 begin the first of May and continue until about the first of November; 

 between Tampico and Monterey they begin about the last of May 

 and continue until October. In northern Mexico, in the State of 

 Chihuahua, the rainy season commences the last of June and ceases 

 in September. The rainfall in southern Mexico is much more abun- 

 dant than in the northern part. Where the wet season is much longer 

 than the dry season the larger rivers contain a good supply of water 

 during the entire year. In the height of the wet season many of 

 them overflow, forming, in depressions, shallow lakes, bayous, and 

 ponds; these later become dry, causing the destruction of a great 

 many fishes. The same conditions hold in the dryer portions, but 

 here many of the streams also become partially dry. On the plateau 

 the water in the dry season in many streams is confined to a portion 

 of the upper part of its course, and such streams usually contain a 

 small number of species of fishes. During the wet season many of 

 the isolated streams in northern Mexico form at their mouths lakes 

 of considerable size, such as Lago de Guzman at the mouth of the Rio 

 Casas Grandes, Lago de Patos at the mouth of the Rio Carmen, etc., 

 etc. From the upper part of these river courses the lakes into which 

 they flow become stocked with fishes. There is considerable alkali 

 in the beds of these lakes which the water takes up in solution. As 

 the dry season comes on the lakes gradually become smaller and the 

 water in them more alkaline. When each lake is reduced to about 

 three-fourths of its original size the water becomes so charged with 

 alkali that the fishes die in great numbers. During the latter part of 

 the dry season there is very little water in the Rio Casas Grandes 

 below Terrasas, more than half of its bed being dry. What is true 



