XXVl INTRODUCTION. 
eastern Mexico; and two more are known from the Rio Panuco, one of these extending 
into Southern Vera Cruz. The southernmost species is Amiurus meridionalis from the 
Rio Usumacinta. The four remaining species occur respectively in Arizona and Sonora, 
Chihuahua and Durango, the Rio Lerma, and the Rio Balsas.. 
The family Lorrcariip&, with over 200 species, ranges from Panama to Montevideo, 
and has evidently evolved in South America from the Siluride. 
The Nearctic Region. 
For fresh-water fishes the voleanic chain of mountains which 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 (Cichlosoma beani) 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 
meaicanus), which last has spread from the Rio Grande on to the plateau north of the 
Lerma System. ‘These only counterbalance the nearctic types which occur south of 
the Nearctic Region, viz. Goodea whitii, Amiurus balsanus, Notropis boucardi, and 
N. aztecus in the Rio Balsas, Amiurus meridionalis and Ictiobus meridionalis in the 
Rio Usumacinta, the latter also in the Rio Papaloapam, and Lepidosteus tropicus, 
ranging 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 Palearctic Region, whilst the latter 
shows relationship only to the Ethiopian. In addition to partly marine groups, such 
as the Salmonide and Gastrosteide, there are several families of true fresh-water fishes 
common to the Palearctic and Nearctic Regions (e. g., Esocidee, Cyprinide, Percide, 
&c.). The latter is characterized by some endemic families (Hiodontide, Amblyopside, 
Percopside, and Aphredoderide), by the numerous dwarfed Percide (Etheostoma, &c.) 
and the great development of the Cyprinodontine (partly marine), and by three groups 
of fishes which are almost peculiar to this region, viz. Catostomine, Siluride of the 
genera Amiurus, Noturus, &c., and Centrarchide. The first two are also found in 
* Cichlosoma labridens, C. bartoni, C. steendachneri, Herichthys cyanoguttatus, and H. pavonaceus. 
