CHAP. XIV.] 
THE NEOTROPICAL REGION. 
55 
enumerated; and 17 of these are found in streams flowing into 
both the Atlantic and Pacific Oceans. On the whole, 11 families 
are represented among the fresh-water fish, and about 38 genera. 
Of these, 14 are specially Nearctic , — Amiurus (Siluridse) ; Fundu- 
lus (Cyprinodontidse) ; Sclerognathus (Cyprinidse) ; and Lepulosteus 
(G-anoidei). A much larger number are Neotropical ; and several 
Neotropical genera, as Keros and Poecilia, are more largely 
developed here than in any other part of the region. There are 
also a considerable number of peculiar genera; — Petenia , Thevaps, 
and Neotrophus (Chromides) ; AElurichthys (Siluridf©) ; Chalci- 
nopsis (Characniidae) ; Characodon, Belonesox, Pseudoxiphophorus, 
Platypcecilus , Mollienesia, and Xiphophorus (Cyprinodontidae). 
A few peculiar Antillean forms are also present ; as Agonostoma 
(Mugilidse) ; Gambusia and Girardinuus (Cyprinodontidse). The 
other families represented are Percidae (1 genus) ; Pristopomatidae 
(2 gen.) ; Gobiidae (1 gen.) ; Clupeidae (2 gen.) ; and Gymnotidae 
(1 genus). 
On the whole the fish-fauna is typically Neotropical, but with 
a small infusion of Nearctic forms. There are a considerable 
proportion of peculiar genera, and almost all the species are 
distinct from those of other countries. The predominant family 
is that of the Cyprinodontidae, represented by 12 genera ; and 
the genus Keros (Chromidae) has here its maximum development, 
containing between thirty and forty species. Dr, Gunther con- 
siders that a number of sub-faunas can be distinguished, corre- 
sponding to some extent, with the islands into which the country 
would be divided by a subsidence of about 2,000 feet. The 
most important of these divisions is that separating Honduras from 
Costa Rica, and as it also divides a very marked ornithological 
fauna we have every reason to believe that such a division must 
have existed during the latter portion of the tertiary epoch. 
We shall find some farther evidence of this division in the 
next class. 
Insects . — The butterflies of various parts of Central America 
and Mexico, having been largely collected, offer us some 
valuable evidence as to the relations of this sub-region. Their 
general character is wholly Neotropical, about one half of the 
