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 ISTearctic, — Aviiurus' (Siluvidad) ; Fundu- 

 lus (Cyprinodontidae) ; Sderognathus (Cyprinida3) ; and Lepidostevs 

 (Ganoidei). A much larger number are Neotropical ; and several 

 Neotropical genera, as Hcros 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, Theraps, 

 and Neotrophus (Chromides) ; ^luricJithys (Siluridae) ; Chalci- 

 nopsis (Characniidse) ; Characodon, Belonesox, Pseudoxiphophorus, 

 Platypcecilus, Mollienesia, and Xiplwphorus (Cyprinodontidae)^ 

 A few peculiar Antillean forms are also present ; as Agonostoma 

 (Mugilidse) ; Gambusia and Girardinuus (Cyprinodontidae). 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 Heros (Chromidae) has here its maximum development, 

 containing between thirty and forty species. Dr. Giinther 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 Eica, 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 



YOL. II.— 5 



