XXVI INTEODUCTION. 



eastern Mexico ; and two more are known from the Rio Paniico, one of these extending 

 into Sonthern Vera Cruz. The southernmost species is Amiurus meridionalis from the 

 Rio Usumacinta. The four remaining species occur respectively in Arizona and Sonera, 

 Chihuahua and Durango, the Rio Lerma, and the Rio Balsas. 



The family Loricariidje, with over 200 species, ranges from Panama to Montevideo, 

 and has evidently evolved in South America from the Siluridne. 



The Nearctic Bcgion. 

 For fresh-water fislies the volcanic chain of mountains whicli stretches across Mexico 

 from Colima nearly to Vera Cruz may be taken as the boundary between the Nearctic 

 and Neotropical Regions. This range has proved an insuperable obstacle to the north- 

 ward migration of the neotropical fishes, none of which have surmounted it to reach 

 the Mexican plateau ; in the narrow lowland strip near the west coast of Mexico one 

 neotropical type [Ckhlosoma bcani) has extended northwards to the Rio Presidio in 

 Sinaloa, whilst in the plains of Northern Vera Cruz and Tamaulipas on the east some 

 neotropical fishes are found, viz. five Cichlids * and a Characinid [Tetragonopterus 

 mexicanus), which last has spread from the Rio Cirande on to the plateau north of the 

 Lerma System. These only counterbalance the uearctic types which occur south of 

 the Nearctic Region, viz. Goodea ivliitii, Aininrus lalsanus, Notrojpls houcardi, and 

 X. aztecus in the Rio Balsas, Amiurus meridionalis and Ictiohus meridionalis in the 

 Rio Usumacinta, the latter also in the Rio Papaloapam, and Lejndosteus tropicus, 

 ransing from Guatemala to Panama. 



The Nearctic and Neotropical Regions are quite distinct, and show no affinity what- 

 ever. The former has much in common with the Paltearctic Region, whilst the latter 

 shows relationship only to the Ethiopian. In addition to partly marine groups, such 

 as the Salmonidie and Giastrosteidaj, there are several families of true fresh-water fishes 

 common to the Pala^arctic and Nearctic Regions (e. g., Esocid;e, Cyprinida% Percid;r, 

 &c.). The latter is cliaracterized by some endemic families (Iliodontida?, Amblyopsida?, 

 Percopsidct', and Aphredoderidie), by the numerous dwarfed Percida? {Etheostoma,8cc.) 

 and the great development of the Cyprinodontin;c (partly marine), and by three groups 

 of fishes which are almost peculiar to this region, viz. Catostominac, Siluridas of the 

 genera Arjiinrns, Noiurus, &c., and Centrarchid;c. The first two arc also found in 



■* GwMosonia labridcii.'i, C. harloni, C. stctiidacJtiieri, llcrichtliijs cijanoijuUaias, and U, pavonaccus 



