REPTILIA, BATRACHIA, AND PISCES. 109 
isolated lakes of the states of Michoacan and Mexico, constitute an important system 
with a very characteristic fish-fauna formed almost entirely of endemic species. Only 
three families of Nearctic fishes are present; the Catostomide are represented by a 
single species of Mowostoma and the Amiuride by an Amiurus, whilst there are 
a dozen Cyprinide, nearly all belonging to endemic genera. The majority of the 
fishes belong to two groups, each with about a score of species, and each belonging to 
a family that includes a number of marine fishes that enter fresh water. ‘These two 
groups are characteristic of the Lerma System and barely overstep its limits; one is 
the somewhat heterogeneous Atherinid genus Chirostoma, the other is the Cyprinodont 
subfamily Characodontin, which includes five well-marked genera, and differs from 
the Funduline in that its species are viviparous. 
The Lerma System has evidently long been isolated, and has been a centre of 
evolution. The chain of volcanoes that border the Mexican plateau to the south have 
barred invasion on that side, and from the north only a few nearctic types have 
reached the Lerma. The main elements of its fish-fauna, Chirostoma and the Chara- 
codontine, have in all probability evolved each from a single ancestral type that 
entered from the sea, from which it is now separated by inaccessible falls. 
The Rio Balsas lies to the south of the Mexican plateau, and is a large river that 
flows into the Pacific; it has not been thoroughly investigated, but appears to have 
a comparatively poor fish-fauna. In addition toa Lerma type (Goodea) that has got 
into the headwaters of its northern tributaries, there is a Cat-fish (Amiwrus) and a 
Cyprinoid (Notropis) ; two neotropical species balance these. The high mountains 
that bound the Balsas System have evidently rendered immigration a difficult feat. 
It is in the lowlands of the Atlantic slope that conditions have been more favourable 
to migration. Lepidosteus, absent from the Mexican plateau and from the Balsas, has 
reached Panama, and a species of Ictrobus (Catostomidee) and an Amiurus occur in 
the Usumacinta, balancing the Cichlid and Characid species of the Rio Grande. 
Dispersal of Neotropical and Nearctic Fishes compared.—tIt has been 
shown above that comparatively few elements of the rich neotropical fish-fauna extend 
north of the Isthmus of Panama, and that of these only the Cichlide exhibit much 
diversity and specialization. In Central America and in rivers of the Atlantic slope 
of Mexico south of Vera Cruz the fish-fauna is almost exclusively neotropical, but 
only two neotropical types have reached the Balsas, none have surmounted the volcanic 
chain that borders the Mexican plateau, and only half a dozen have penetrated north 
of Vera Cruz in the Atlantic coast-streams of northern Mexico. In these streams 
there are many nearctic species, but in Atlantic rivers south of Vera Cruz only three. 
The Rio Grande exhibits a paucity of nearctic types as compared with the Mississippi, 
and this is still more marked in the Lerma System, which possesses a peculiar endemic 
fish-fauna. In the Balsas, equally isolated from the north and the south, nearctic and 
neotropical species balance, but there are only two or three of each. 
