108 ORIGIN, ETC., OF THE FAUNA. 
for the systematist ; the species are closely related and the genera ill-defined. ‘They 
give the impression that their number and variety, as compared with the Characide 
and the Pimelodide of this area, are not due to an earlier migration, but to a greater 
capacity for differentiation and a more rapid evolution. 
The Nearctic Region.—North America has a fish-fauna as different as possible 
from that of South America. The Ostariophysi are represented by the Catostomide 
(Suckers) and by a family of Cat-fishes, the Amiuride, each restricted to this region 
except for one or two species in China; in addition, there are a number of Leuciscine 
Cyprinide, a group well represented in Eurasia. The Perches belong to the endemic 
family Centrarchide and to the Percide. The Cyprinide, Percide, Esocide, and 
Umbride are holarctic, but there are several endemic families, Hiodontide, Per- 
copside, etc., in addition to those mentioned above, that make this region quite 
distinct from the Palearctic. 
Origin of Nearctic Fishes.—Numerous fresh-water fishes have been described 
from early Tertiary deposits in North America. Lepidosteide and Amiide were 
already present in the Upper Cretaceous or Basal Eocene of North America and 
Europe. An interesting assemblage is known from the Lower Eocene (Green River 
shales) of Wyoming, including Percopside or Aphredoderide (Amphilaga and Erisma- 
topterus), Percide (Mioplosus), and Centrarchide (Priscacara), as well as the Osteo- 
glossid Dapedoglossus; there are some other fishes that may have been marine as well 
as fresh-water, e. g. Xiphotrygon (Trygonide), Notogenus (Chanide), Diplomystus 
(Clupeide), and Rhineastes (Ariide). Catostomide are also known from the Eocene 
and Amiurus from the Lower Miocene. | 
The known history of the Nearctic fishes is in harmony with the supposition that 
North America separated from Eurasia at the end of the Cretaceous and developed 
several of its endemic types during the Eocene, and that since the Eocene one or 
more connections with Eurasia have brought Leuciscine Cyprinide to America, and 
have established two or three Catostomids and Amiurids in China. The identity of 
some of the more northern species of the two continents shows that a connection 
across the Behring Sea must have persisted until a recent date. 
Dispersal of the Nearctic Fishes.—In the Mississippi all the characteristic 
nearctic types are represented; to the north, west, or south there is a marked 
impoverishment. In the Rio Grande, Hiodontide, Percopside, Apbredoderide, 
Umbride, and Esocide are absent, and the Percide and Centrarchide are reduced to 
a few species only ; there are several Cyprinids and Catostomide. Isolated streams 
and lakes of Chihuahua and Durango, the portions of the Yaqui and Mezquital Rivers 
east of the Sierra Madre, and the rivers of Tamaulipas and northern Vera Cruz have a 
fish-fauna essentially similar to that of the Rio Grande. On the southern part of the 
Mexican Plateau, the Rio Grande de Santiago or Lerma above the falls, with the 
