MAY, 1902. THE ICHTHYOLOGY OF MKXIO MKKK. 125 



basins, but in each it has a near relative. The faunas of the 

 Valley of Mexico, the Rio Moctezuma near its source, and the 

 Lerma Basin are practically the same. Jordan and Snyder 

 examined the Rio Verde, a tributary of the Rio Panuca, near 

 Rascon, San Louis Potosi; none of the species taken there have 

 been taken in either the Valley of Mexico or the Rio Moctezuma 

 at San Juan del Rio, or the Rio Lerma, all of which have pract- 

 ically the same fauna, aad one peculiarly characteristic of this 

 region. Ten of the 18 genera known from the Lerma Basin have 

 no representatives yet known from other waters. Chirostoma has 

 not been taken in the adjacent water systems. 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. The other five genera, Lampetra, 

 Ameiurus, Moxostoma, Notropis and Hybopsis are northern genera, 

 and all except Notropis are not represented by any species farther 

 south than the Rio Lerma. 



Of the 46 species found in this region, 30 belong to two fami- 

 lies; 16 to Pcfdliidce and 14 to Athcrinida. It is curious to note 

 here that all of the Pa'ciliida are viviparous, and only one species, 

 Gambusia infans, has the anal fin of the male placed well forward 

 and modified into an intromittent organ such as is characteristic 

 of Heterandia, P&'ci/ia and the like. In the other 15 species the 

 anal fin of the male has its normal position and size. It is 

 slightly modified by the shortening of the first five or six rays, 

 and their slight separation from the rest of the fin by a shallow 

 notch. Just what part this fin plays in fertilizing the eggs in 

 the body of the female is not known. It evidently plays a promi- 

 nent part in this operation. 



The spawning time for these fishes is near the close of the dry 

 season. At this time the water is more concentrated, as is also 

 the food on which the young must feed. The aquatic insects, crus- 

 taceans and small fishes which would feast on the eggs if deposited 

 then are also more concentrated, so that depositing eggs at this time 

 would mean considerable destruction to the species. As it is, the 

 young are born in a well developed stage, and have time to reach 

 some size before the wet season sets in. They are then, perhaps, 

 in the best condition to become widely distributed as the volume 

 and area of water increases. As the dry season approaches again, 

 and small streams and ponds become dry, many of these small 



