Introduction. li 



in the Lerma basin; indeed, no other river in North America has so 

 large a proportion of its fishes belonging to a salt-water* family. It 

 is probable that when this basin is more thoroughly explored the 

 number will be considerably increased. I had seen but few speci- 

 mens of Chirostoma before going to Mexico, and thus had never had 

 an opportunity to study these fishes. And while I was careful to pick 

 up specimens of all species observed, yet my unfamiliarity at that 

 time with the group caused me, no doubt, to overlook some species. 

 Again, there are a number of small isolated lakes which have never 

 been visited. It is known that some of these lakes, as Patzcuaro 

 and Ziratiuen, have in them one or more characteristic species, and 

 no doubt most of the others also have. The Lerma River system is 

 far from being thoroughly explored, but apparently its fishes are quite 

 as distinct and characteristic as if the fauna were insular. This area 

 has been in a center of distribution. 



The Rio Balsas is one of the largest rivers in Mexico. It is south- 

 east of the Lerma, and drains about an equal area; and though these 

 two rivers are so near each other, only one species is known to be 

 common to them. But one species of silversides and four of killi- 

 fishes are known from the Rio Balsas, yet these two families comprise 

 two-thirds of the fishes of the Lerma basin. Only the upper and 

 northern tributaries of the Rio Balsas have been explored ; however, 

 enough has been done to indicate the nature of its fauna and that 

 it contains comparatively few species of fishes. 



The South and Central American faunas prevail largely as far 

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

 north apparently keep to the lowland streams; especially is this true 

 on the Pacific side. The most northern representative of the south- 

 ern American fauna, one of the Cichlids, is found at Mazatlan. On 

 the east coast this family has a representative in Texas. 



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 streams never sustain 

 a large number of species of fishes. On the Mexican plateau the 

 largest and most important lakes are found in the Lerma basin ; Lake 

 Chapala, being the largest and the only one which has a river outlet 

 and inlet, sustains the largest fish fauna. Patzcuaro, a large lake 

 with no inlet nor outlet, does not have so varied a fauna, but supports 

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

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



*The Chirostoma are the only fishes belonging to a salt-water family found 

 on the Mexican plateau. 



