INTRODUCTION. xlix 



Falls is also the dividing line between the fauna of the upper Snake 

 River and of that portion of it below the falls. In both of these in- 

 stances fishes belonging to the fauna above the falls have not become 

 established below them. 



It is quite probable that along the middle course of these Pacific 

 slope rivers in Mexico there are no fishes, or possibly a few species 

 of PoeciliidcB or of some brackish or salt water form which was asso- 

 ciated with the rivers in their formation. Many fishes no doubt go 

 over falls and cascades, but not in quantities sufficiently large to 

 enable the survivors to become established below them. A study of 

 the fishes along the courses of these Pacific coast streams would be 

 very interesting indeed. 



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, is a tributary 

 of the Rio Grande de Santiago, which flows into the Pacific. Judging 

 from the nature of its fish fauna, the Valley of Mexico was formerly 

 a part of the Lerma drainage system. The fish fauna of this region 

 is very different from that either to the north or the south. From 

 the area which includes the valley of Mexico the head waters of the 

 San Juan del Rio and the Lerma basin, there are at present fifty-four 

 species of fishes known, only two of which, Myzostoma austrinum 

 Bean, and Aztecula vittata (Girard), have been taken in any other 

 river basin. These fifty-four species belong to twenty-one* genera, 

 eight of which are peculiar to this region. 



Of the genera found elsewhere and which occur on the plateau, 

 Characodon is represented in southern Mexico, central America, and 

 Lower California ; Gambusia comprises a number of small viviparous 

 fishes usually inhabiting swamps and springs all the way from 

 southern Illinois to Panama ; Goodea is represented in the Rio Panuco 

 and the Rio Balsas, and Aztecula in the Rio Balsas; one species of Cki- 

 rostoma is found in the Rio Mezquital; Lampetra, Amiurus, Myzostoma, 

 Nototropis and Hybopsis are northern genera, and all except Noto- 

 tropis are not represented by any species farther south than the Rio 

 Lerma. Thirty-six of the fifty-four species found in this region belong 

 to two families, twenty to Pceciliidce (the killifishes), and sixteen to 

 AtherinidcB (the silversides). It is curious to note here that all of 

 the killifishes are viviparous, yet only two species, Gambusia infans 



*The genera in italics are peculiar to this region. 



Lampetra i, Amiurus i, Myzostoma i, Xystrosus i, Algansea 4, Falcula i, 

 Aztecula 3, Nototropis i, Evarra 2, Hybopsis i, Zoogoneticus 6. Girardinichthys 

 i, Characodon 4, Chapalichthys i, Gambusia i, Goodea 2, Skiffia 4, Poscilia i, 

 Chirostoma 16, Agonostomus i, Cichlasoma i. 



