No. 443 ] FRESH-WATER FISHES OF MEXICO. 779 



in any other river. These 49 species belong to 17 1 genera, 10 

 of which arc peculiar to this region. 



Of the genera found elsewhere, 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 49 species found in this 

 region, 33 belong to two families; 17 to Pceciliidae (the Killi- 

 fishes) and 16 to Atherinidae (the Silversides). It is curious to 

 note here that all of the killifishes are viviparous, yet only one 

 species, Gambusia infaus Woolman, has the anal fin of the male 

 placed well forward and modified into an intromittent organ such 

 as is characteristic of Heterandia. Pcecilia 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. This modification was first noticed by 

 Gunther in Characodon lateralis Gunther. It was also described 

 by Bean in Zoos-one tints robust us (Bean), and by Jordan and 



attached to it. Just what part this fin plays in fertilizing the 

 eggs in the body of the female is not known, but it evidently 

 plays a porminent part in this operation. 



I was fortunate to collect these fishes during the breeding 

 season and so their viviparity was easily proved. The largest 

 killifish known from the Lerma Basin reaches a length of 8 or 

 10 inches. The accompanying figures are made from a photo- 

 graph of the largest female of this species I was able to obtain. 



find its way into a distant museum and this explains its rather 

 dilapidated appearance. The ovary consists of a membranous 



1 The genera in italics are peculiar to this region. Aztecu , a , No| js , 



Xystrosus 1, Evarra 2, Falcula 1, Ilybopsis 1 . Zoogoneticus 5. Girardinichthys 

 i, Characodon 2, Chapalichthys 2, Gambusia 1, Goodea 7. Chirostoma 16. 



