262 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



rier in Cretaceous time as they are to-day, a proposition fully as tenable 

 as the opposite hypothesis that they were united. 



Evidences of Marine Connection in the Tertiary Period. — The inde- 

 pendent opinions of Agassiz, Dall, Verrill, Moore, Etheridge, Gabb, Scott, 

 and Bland all tend to conclude that the oceans were in some manner 

 united in that portion of the Tertiary period prior to the close of the 

 Miocene. In fact, all who have discussed the question from a biologic 

 standpoint, with the exception of Jukes-Browne, place the date of the 

 connection of the two oceans across Tropical America in early Tertiary 

 time. 



The researches detailed in the first part of this paper present the first 

 data by which these deductions can be tested in the light of the geologic 

 history, and enable us to fix with greater exactitude the date of the two 

 most important events in Tropical American history, — the epoch of 

 geologic time when common species lived in both the Pacific and Carib- 

 bean waters (the date of the union of the waters), and the epoch when 

 they were completely and forever separated by the great Caribbean 

 Tertiary orogenic revolution. 



In discussing this period, however, we are at first confronted with the 

 indisputable fact that all the known Tertiary sediments of the Central 

 American and Isthmian region are Atlantic sediments, and that no strat- 

 igraphic proof has as yet been discovered by which marine connection 

 can be estabished. 1 Hence all deductions must be limited to paleonto- 

 logie evidence. 



It is an interesting fact that, around the great curve of the Caribbean 

 and thence eastward along the northern coast of Colombia and Vene- 

 zuela, the older Tertiary formations are all of the same lithologic aspect 

 as those bordering the Atlantic and Gulf States of the North American 

 continent. 



If the early Tertiary littoral completely bordered the Yucatan pro- 

 eontinent, then marine waters existed between the Chiapas-Yucatan 

 mainland and the Western Antilles, and no bridge between them and 

 the Antilles could have possibly existed at that time. Upon this prob- 

 lem we are not at present prepared to state an opinion. It is impos- 



1 Dr. J. W. Spencer has recently inferred Pleistocene connection across the 

 Isthmus of Tehuantepec. His deductions are based upon the occurrence of gravel 

 covered streamways in the summit divide of Cretaceous rocks which separate the 

 lower lying coastal formations of later date. No evidence is presented, however, 

 to show the marine affinities of these features, and from his data our conclusions 

 would he that the Tehuantepec Isthmus has remained land since its earliest origin. 

 See Bull. Geol. Soc. of America, Vol. IX. pp. 13-34, Rochester, 1897. 



