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 
conneetion of the two oceans across "Tropical America in early Tertiary 
time. 
Тһе 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.! Hence all deductions must be limited to paleonto- 
logic 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- 
continent, 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 be that the Tehuantepec Isthmus has remained land since its earliest origin. 
See Bull. Geol. бос. of America, Vol. IX. pp. 19-94, Rochester, 1897. 
