FRESHWATER FISH FAUNA 



205 



pharodon (California), Mylocyprinus and Mylocheilus (fossil, Idaho; 

 Recent, Columbia River), and Mylopharyngodon (China). The indi- 

 cated similarities in this series pertain to the common possession of 

 crushing type (molariform) pharyngeal teeth, an obvious feeding 

 adaptation subject to independent and repeated evolution through- 

 out the family. More likely candidates are the western genus Gila 

 and Tribolodo7i of the Japanese fauna. None of the affinities postu- 

 lated above has been thoroughly investigated. At the present time, 

 only one American genus, Gila, is regarded by some as congeneric 

 with an Old World genus, Phoxinus (see Berg, 1949, p. 571), but I 



Fig. 14. Redside shiner, Richardsonius balteatus, of the Columbia 

 River basin. Fish of this type represent the earliest known fossils of the 

 Cyprinidae in North America. (From Jordan and Evermann, 1900, 

 Fig. 105). 



do not accept this allocation for reasons already given. The New 

 World distribution of the Cyprinidae (Fig. 13) indicates that this 

 family is less tolerant of low maximum temperatures than are 

 the Catostomidae. 



The three secondary families have barely been able to invade the 

 western fauna from the south and southeast. One species of mojarra 

 (Cichlasoma beani), of the tropical family Cichlidae, has managed to 

 reach the Yaqui River; the genus to which it belongs is most speciose 

 in Middle America, but was derived from a South American ancestor 

 (Regan, 1906-08, p. xiii). The viviparous topminnows of the family 

 Poeciliidae, exclusively American and essentially tropical, have 

 moved a little farther north to the lower Colorado River system, 

 where they are represented by a single species of Poeciliopsis in 

 southern Arizona (two species occur in the Yaqui). The egg-laying 

 killifishes of the family Cyprinodontidae, largely tropical but push- 



