402 DR. GUNTHER ON THE FISHES OF CENTRAL AMERICA. 
exception of a small Porites and Stephanocora; that these and other differences do not 
favour the theory entertained by some geologists, viz. that there has been a communi- 
cation between the two oceans at this point, and that the Gulf-stream flowed across the 
isthmus into the Pacific, within comparatively recent geological times. 
It is not within the scope of this paper further to discuss the point on which Messrs. 
Duncan and Verrill are at variance, as we cannot assume that the present fish-fauna 
existed at so early a period. From the observations made on the fishes and shells we 
are obliged to conclude that down to a very recent period a connexion between the two 
seas has been kept open by channels and straits wide enough to allow of the passage 
of these animals. Why corals, or at least a part of them, should not have been dis- 
persed by their floating germs in a similar manner, is a circumstance which we cannot 
explain. 
The occurrence of identical species of freshwater fishes in rivers running to the two 
opposite oceans is a matter of much less difficulty, and, besides, has been very generally 
observed in various parts of the globe. ‘The same agencies which in other countries have 
effected a wider dispersion of one species than of another must have been at work here 
also. Prof. M. Wagner has, in his Memoir quoted above, so fully treated of this part of 
our subject, with particular reference to the hydrographical peculiarities of the isthmus, 
that we need not dwell further on it. 
§ 7. Definition of the Characteristics of the Fish-fauna of Central America. 
In defining the zoological characters of Central America, expressed in its fish-fauna, I 
confine myself to the freshwater fishes proper. Here the nearctic types become extinct, 
and are represented by five generic types, four of which, although with numerous species 
in the north, have but a single one here—Lepidosteus, Amiurus, Sclerognathus, and 
Haplochilus. Fundulus, extending a little further southwards (with one species in 
Western Ecuador), is represented by four species in Guatemala. Not one of these 
species is identical with a North-American. 
Much greater is the affinity with neotropical types; and their representatives are much 
more numerous: there is one species of Acara, one of Macrodon, seven of Tetragono- 
pterus, one of Anacyrtus, twelve of Pimelodus, one of Plecostomus, two of Chetostomus, 
two of Loricaria, one of Anableps, one of Carapus, the latter being identical with a 
species from Guiana. Types in common with the West-Indian Islands are—Agono- 
stoma with three species (one of which is said to be identical with a Jamaican species), 
Girardinus and Gambusia with one, the two latter genera being also represented in the 
Southern States of North America. The Siluroid genus Arius, which extends over the 
tropics generally, is represented by nine species. 
Finally, the following genera are peculiar to Central America, or at least have attained 
there to the greatest development :—Heros and the allied Neetroplus and Petenia with 
thirty-four species, Mlurichthys with two, Chalcinopsis with three, Characodon with one, 
