402 DE. GtJNTHEE ON THE FISHES OF CENTRAL AMERICA. 



exception of a small Pontes 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 Venill 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 oi 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 pVoper. Here the nearctic types become extinct, 

 and are represented by five generic types, four of which, although with numerous species 

 m the north, have but a single one here — Lepidosteus, Amiurus, Sclerognathus, and 

 Haplochilus. Fmidtclus, extenduig 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 theii- 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 Chcetostomus, 

 two of Loricaria, one of AnaUeps, 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, jElurichthys with two, Chalcinopsis with three, Characodoii with one, 



