FRESHWATER FISH FAUNA 195 



of volcanoes lyin^i? at about 19° N. Lat., a sucker {Idiohiis meri- 

 dionalis) and a catfish {Ictaluriis meridionalis) have managed to 

 reach the basin of the Rio Usumaclnta, Guatemala. The Mexican 

 coastal regions, particularly the Atlantic coastal plain, have simi- 

 larly allowed the northward penetration of Neotropical groups. A 

 single representative each of the characins {Astyanax jasciatus) and 

 cichlids {Cichlasoma cyanoguttatiim) has invaded the United States 

 in extreme southwestern Texas, and the characin has moved into 

 New Mexico (these two are the only primary families shared by the 

 United States and South America). 



The southern end of the Middle American peninsula has been 

 invaded by a number of South American groups, most of which 

 drop out beyond western Panama, although the eel-like Gymnotidae 

 are represented as far north as Guatemala. The fish fauna inhabit- 

 ing the peninsular-like region between the Isthmus of Tehuantepec 

 and Panama is dominated by secondary freshwater fishes, notably 

 the Poeciliidae and the Cichlidae. The viviparous poeciliids pre- 

 sumably arose within this area, diversified greatly, and spread 

 northward and southward ; the cichlids and pimelodids, on the other 

 hand, probably originated in South America and speciated in the 

 unsaturated environment of Middle America. These two groups, 

 with additions from the Characidae and Atherinidae, constitute the 

 more tropical portion of Middle American fauna. Intervening be- 

 tween this portion of the Middle American fauna and the Nearctic 

 fauna of northern Mexico are several transitional faunas and the 

 highly distinctive Lerma fauna, which includes a limited representa- 

 tion of both middle American and North American genera, and is 

 dominated by the endemic cyprinodont family Goodeidae and the 

 endemic and diverse atherinid genus Chirostoma. The Lerma fauna 

 may be regarded as a distinct element in the Middle American 

 complex. 



The relatively impoverished primary fauna of western United 

 States is indicated for the 8 drainage areas shown on the map (Fig. 

 1). Within the entire area there are only 6 primary families (Table 

 I): the mudminnows (Umbridae), minnows (Cyprinidae), suckers 

 (Catostomidae) , catfishes (Ictaluridae), trout-perches (Percopsidae), 

 and sunfishes (Centrarchidae). The first three are regarded as of 

 Eurasian origin, and the last three as North American. 



The mudminnows (Fig. 3) are represented by two genera, Nov- 



