HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 269 



By elimination, we have concluded that the only period of time 

 since the Mesozoic within which communication between the seas could 

 have taken place is the Tertiary period, and this must be restricted 

 to the Eocene and Oligoeene epochs of that period. The paleontologic 

 evidence upon which such an opening can be surmised at this period 

 is the occurrence of a few Californian Eocene types in the Atlantic sides 

 of the Tropical American bai'rier, within the ranges of latitude between 

 Galveston (Texas) and Colon, which are similar to others found in Cali- 

 fornia. 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 occurrence 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 con- 

 nection at the Isthmus of Panama, and we could just as well maintain 

 that the locus thereof might have been at some other point in the Cen- 

 tral American region. 



The reported fossil and living species common to both oceans are lit- 

 toral forms, which indicate that if a passage existed it must have been of 

 a shallow and ephemeral character. 



There is no evidence either from a geologic or biologic standpoint for 

 believing that the oceans have ever communicated across the Isthmian 

 regions since Tertiary time. In other words, there is no evidence for 

 these later passages which have been established upon hypothetical data, 

 especially those of Pleistocene time. 



The numerous assertions so frequently found in literature that the 

 two oceans have been frequently and recently connected across the 

 Isthmus, and that the low passes indicative of this connection still exist, 

 may be dismissed at once and forever and relegated to the domain of 

 the apocryphal. A few species common to the waters of both oceans in 

 a predominantly Caribbean fauna of the age of the Claiborne epoch of 

 the Eocene Tertiary is the only paleontologic evidence in any time upon 

 which such a connection may be hypothesized. 



There has been a tendency in literature to underestimate the true 

 altitude of the Isthmian passes, which, while probably not intentional, 

 has given encouragement to those who think that this Pleistocene 

 passage may have existed. Maack 1 has erroneously given the pass 



1 Harper's Magazine, 1873, p. 803. 



