INTRODUCTION. XXIiX 
All the Cyprinodontide and Atherinide 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 specific characters 
appear to be adaptive—e. g., the lower jaw is short in C. promelas and strongly 
projecting in C. ocotlane, whilst C. sphyrena is remarkable for its strong dentition ; 
such characters may have arisen during physiological isolation. 
The Neotropical Region. 
The families Lepidosirenide, Characinidee, Cichlide, and Polycentride are either 
almost or entirely restricted to the Neotropical and Ethiopian Regions, as are the 
Siluride of the groups Pimelodine and Doradine. 
The Neotropical Region is especially characterized by the great development of the 
Characinide and Siluride, and by the presence of endemic families derived from them. 
The Characinide are represented by an extraordinary variety of endemic types, and 
they have given rise to the Gymnotide; the neotropical Siluride include several 
sub-families peculiar to this region, and from them the endemic families Loricariide 
and Aspredinide 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 Cichlide and Cyprinodontide. 
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, Amiurus, Notropis) and by the scarcity or absence of typical 
Central-American groups (Cichlide represented by a single species; Pimelodine, 
Belonesox and Xiphophorus absent). 
The Rio San Juan, with Lakes 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, Ictiobus, and Lepidosteus), by numerous Cichlide of the genus Cichlosoma, 
