Isthmus Barriers Separating Fish Faunas 277 



existed has been long enough to allow the fishes of the two sides 

 to become practically distinct. That the mollusks of the two 

 shores are almost wholly distinct, as shown by Dr. Fischer, is 

 even stronger evidence of the remoteness of the time when the 

 means of communication between the two oceans could have 

 existed, for ' species ' among the mollusks are probably more 

 persistent than among fishes. 



"Our present knowledge, therefore, of the fishes of tropical 

 America justifies us in regarding the fish faunas of the two coasts 

 as being essentially distinct, and believing that there has not 

 been, at any comparatively recent time, any waterway through 

 the Isthmus of Panama." 



It is thus shown, I think, conclusively, that the Isthmus of 

 Panama could not have been depressed for any great length 

 of time in a recent geological period. 



Conclusions of Dr. Hill. These writers have not, however, con- 

 sidered the question of generic identity. To this we may find 

 a clue in the geological investigations of Dr. Robert T. Hill. 



In a study of "The Geological History of the Isthmus of 

 Panama and Portions of Costa Rica," Dr. Hill uses the follow- 

 ing language: 



"By elimination we have concluded that the only period 

 of time since the Mesozoic within which communication be- 

 tween the seas could have taken place is the Tertiary period, 

 and this must be restricted to the Eocene and Oligocene epochs 

 of that period. The paleontologic evidence upon which such 

 an opening can be surmised at this period is the occurrence of a 

 few California Eocene types in the Atlantic sides of the tropical 

 American barrier, within the ranges of latitude between Gal- 

 veston (Texas) and Colon, which are similar to others found in 

 California. There are no known structural data upon which 

 to locate the site of this passage, but we must bear in mind, 

 however, that this structure has not been completely explored. 



" Even though it was granted that the coincidence of the oc- 

 currence of a few identical forms on both sides of the tropical 

 American region, out of the thousands which are not common, 

 indicates a connection between the two seas, there is still an 

 absence of any reason for placing this connection at the Isth- 

 mus of Panama, and we could just as well maintain that the 



