THE AMERICAN NATURALIST. [Vol. XXXVII. 



far as known are that of the Rio Grande. From the Rio Nazas 

 are known 12 1 species of fishes, 6 of these (listed in bold type) 

 have been taken in the Rio Grande or its tributaries, all of the 

 others except Stypodon signifcr and Characodon garmani, belong 

 to genera well represented in the Rio Grande. The genus 

 Stypodon is known only from the Rio Xazas. and Characodon is 

 a tropical genus. Every large lake or river, as a rule, contains 

 one or more species of fishes not found in other localities. 



It is evident that the larger number of the Rio Grande fishes 

 have migrated directly or indirectly from the Mi>sissippi valley, 

 23 of its 85 species being found in the Wabash in Indiana. 

 This fauna has crowded its way over the divide and has become 

 more firmly established in the Pacific coast streams of Sonora 

 than has the Colorado river fauna, and one species (Xotropis 

 uigroticniatus) at least has gotten as far south as the Rio Balsas 

 in southern Mexico. 



The southern portion of the Mexican plateau is drained by 

 two rivers: the one to the east, the San Juan del Rio, is a 

 small stream which flows into the Rio Panuco. The other, 

 the Lerma, flows into the Pacific. The Lerma is the longest 

 river in Mexico. The valley of Mexico was formerly a part of 

 the Lerma drainage system. The fish fauna of this region is 



the 7 area which includes the valley of Mexico,* the head waters 

 of the San Juan del Rio ' and the Lerma basin, are known at 

 present 49 species of fishes, not one of which is known to occur 



