INTEODUCTION. xxis 



All the CypnnodontiLlfe and Atherinidge of the Lerma System appear to have evolved 

 therein, in all probability each from a single prototype. 



Of the fourteen species of Chirostoma, at least eleven, and probably all the fourteen, 

 occur together in Lake Chapala ; several of these species are very closely related and 

 it is by no means easy to distinguish between them ; some of the specilic characters 

 appear to be adaptive — e. r/., the lower jaw is short in C. ]^iromelas and strongly 

 projecting in C. ocotlane, whilst C. sphyrxim is remarkable for its strong dentition; 

 such characters may have arisen during physiological isolation. 



The JVeotrojiical Her/ion. 



The families Lepidosirenidie, Characinidte, CichlidiE, and Polycentridic are either 

 almost or entirely restricted to the Neotropical and Ethiopian Regions, as are the 

 Siluridse of the groups PimelodiniB and Doradinse. 



The Neotropical Region is especially characterized by the great development of the 

 Characinidaj and Siluridae, and by the presence of endemic families derived from them. 

 The Characinidae are represented by an extraordinary variety of endemic types, and 

 they have given rise to the Gymnotidfe; the neotropical Siluridie include several 

 sub-families peculiar to this region, and from them the endemic families Loricariidie 

 and Aspredinidee have evolved. 



The Neotropical Region may be held to include three principal divisions, which may 

 be termed the Patagonian, Brazilian, and Central-American Sub-regions. The fish- 

 fauna of Chili and Patagonia is chiefly remarkable for the scarcity of true neotropical 

 types ; in Southern Mexico and Central America several South-American groups are 

 absent, but this sub-region is characterized also by the presence of a few nearctic 

 fishes and by the development of a number of peculiar Cichlidse and Cyprinodontidte. 



Our knowledge of the fresh-water fishes of the Central-American Sub-region is so 

 incomplete that any division of this area into provinces can only be provisional. The 

 Rio Balsas with its tributaries forms a province defined by the presence of some 

 nearctic types {Goodea, Amiarus, Notrojns) and by the scarcity or absence of typical 

 Central-American groups (Cichlidie represented by a single species ; Pimelodina?, 

 Belonesox and Xiphophorus absent). 



The Rio San Juan, with Lukes Managua and Nicaragua, may perhaps be regarded as 

 a province (San Juan Province) characterized by the peculiar Cichlid fauna ; the region 

 to the north of these right up to and including the Papaloapam System forms another 

 province, defined by the presence of three nearctic fishes (viz. one species each of 

 Amiurus, Ictiohus, and Lejjidosteas), by numerous Cichlidfe of the genus Cicldosoma, 



