124 FISHES FROM GUAYMAS — KVERMANN AND JENKINS. 



Istliinus of Pauama permitting tlie passage of tisbes from one side to 

 the other. 



Subsequently, Dr. Giintlier, in liis "Introdnctiou to the Study of 

 Fishes" (1880, p. 280), claimed a still larger proportion of the fishes of 

 tropical America to be identical on the two sides of the continent. He 

 concluded that "with scarcely any exceptions the genera are identical, 

 and of the sijecies found on the Pacific side nearly one-half have proved 

 to be the same as those of the Atlantic. The explanation of this fact 

 has been found in the existence of communications between the two 

 oceans by channels and straits which must have been open till within 

 a recent i)eriod. The isthmus of Central America was then partially 

 submerged, and appeared as a chain of islands similar to that of the 

 Antilles; but as the reef-building corals flourished chiefly north and 

 east of those islands, and were absent south and west of them, reef 

 fishes were excluded from the Pacific shores when the communications 

 were destroyed by the upheaval of the land." 



But of the fifty-nine species which Dr. Gilnther regarded as identical 

 on the two shores, thirty are now regarded as specifically distinct by 

 Dr. Jordan (Proc. LT. S. Nat. Mus. 1885, 394), and this leaves but 15 

 per cent, of the one hundred and ninety-three as common to both coasts. 



Of four hundred and seven species from the two coasts known to Dr. 

 Jordan in 1885, he regarded but seventy-one species, or 17.J per cent., 

 as specifically identical ; and if to this be " added some eight hundred 

 species known from the Caribbean Sea and adjacent shores, we have 

 about G per cent, of the whole number known, as common to the two 

 coasts." 



Upon this evidence Dr. Jordan based his opinion that "fuller investi- 

 gations will not increase the proportion of common species, and, if it 

 does not, the two fauuic show no greater resemblances than the similarity 

 of physical conditions on the two sides would lead us to expect." 



The explorations since 1885 have resulted, (1) in an addition of about 

 one hundred species to one or the other of the two faunte; (2) in show- 

 ing that at least two species that were regarded as identical on the two 

 shores {Githarichthys spilopterufi and G, f/ilberti) are probably distinct; 

 and (3) in the addition of but two species to those common to both coasts 

 {Ha'mnlon Hieindachneri J. «Si G. and Sidera casianea J. & G. of the west 

 coast probably being identical with H. scliranki and GymnotUorax fu- 

 nebris of the east coast). 



All this reduces still further the percentage of common species. 



Of the one hundred and ten species obtained by us, twenty-four, or 

 less than 21 per cent., appear to be common to both coasts. Of these 

 twenty-four species, at least sixteen, from their wide distribution, would 

 need no hypothesis of a former water way through the isthmus to account 

 for their presence on both sides. They are species fully able to arrive 

 at the Pacific shores of the Americas from the warm seas west. It thus 

 appears that not more than eight species, less than S per cent, of our 



