INTEODUCTION. xiu 



fresh-water fishes belong to famihes which are wholly (e. g. Percidae, Cichlidae) 

 or chiefly {e. g. Cyprinodontida;, Silurida-) coniiiied to fresh water. In Mexico 

 and Central America these are the following: — Cichlidae, Percidas, Centrarchidse, 

 Cyprinodontidfe, Loricariidee, Siluridee, Cyprinidie, Gymnotida;, Characinidce, and 

 Lepidosteidse. 



The CiCHLiD^ are Perciform fishes which bear a considerable resemblance in 

 appearance and anatomy to the most generalized group of Perciformes, the widely- 

 distributed marine family Serranida?, but differ from them in certain features of 

 specialization, such as the presence of but a single nostril on each side, the absence 

 of teeth on the palate, the coalescence or sutural union of the lower pharyngeals, and 

 the reduced number of branchiostegal rays. 



No known fossils can be referred to the Cichlidae, which inhabit America, from 

 Texas to Montevideo, and x\frica, including Madagascar. Seven species occur in 

 Syria, and a single genus with three species inhabits Ceylon and Southern India. 



The American Cichlidae comprise over 150 species which may be arranged in 

 23 genera; from Africa more than 200 species referred to 35 genera have been 

 described. 



Not one of the genera is common to Africa and America, but the South-American 

 Acara is scarcely generically distinct from the African Paratilapla, and there can be 

 no doubt that these are the most generalized of living Cichlidae and very near to the 

 ancestral type of the family. 



The Mexican and Central-American Cichlidae are more specialized than the South- 

 American ones, and have certainly been derived from them ; not one of the genera 

 with three anal spines is found north of the Isthmus of Panama, and all the South- 

 American Cichlidae have simple conical teeth. 



Of the Southern types only Ciclilosoma has reached Mexico and Central America, 

 and has there given rise to a variety of more specialized forms. 



We have no evidence in favour of dating the origin of the Cichlidae before the 

 Eocene. At the same time we have to explain their occurrence in South America, 

 Africa, and India at the present day. Boulenger, in his address to the Zoological 

 Section of the British Association in 1905, whilst adopting a non-committal attitude, 

 put forward the hypothesis that the Cichlidae were originally a Northern group, and 

 that in the Eocene they ranged over North America and Northern and Eastern Asia, 

 which Avere then one continent, and that they have attained their present distribution 

 by a southward migration and by becoming extinct in their original habitat. This view 



