4. The Isolation of the Fauna of the Mexican Plateau.
The Lerma, draining the Mexican plateau, harbors an old fauna. Only three of its fifty-four species have been taken in any other river basin. Eight of its twenty-one genera are peculiar to it.
5. Paucity of the Pacific Slope Fauna.
From the Pacific slope of South America, south of Panama and north of Chile, but 55 species of fresh-water fishes have been reported. This condition is not unlike the condition in North America, for more species may be obtained from a single favorable brook in the Mississippi valley than are found on our entire Pacific slope, from San Diego to Alaska.
Fifteen out of the 55 species are also found on the Atlantic slope, and the others, with three exceptions, belong to genera of wide distribution on the Atlantic slope. None of the species would cause any surprise if found on the Atlantic slope.
