204 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
over the areas, richly studded with remains of its animal life, but so 
different from similar life on the Pacific side that not a single fossil 
is common to the sediments of the two oceans. 
The composition of these Lower Cretaceous beds clearly exhibits the 
fact that this subsidence deepened towards the southeast. Twenty-five 
thousand feet of limestones in eastern Mexico near the Tropic of Cancer, 
as seen by McGee and the writer, testify to. the fact that it there equalled 
more than 6,000 fathoms, or as much as some of the greatest depressions 
yet discovered in the oceans, and yet there is no evidence in the Creta- 
ceous faunas that the Pacific barrer was broken. 
What happened in the Windward regions during Lower Cretaceous 
time can be only hypothetically conceived. Such a subsidence increas- 
ing southeastward may have been at least so far reaching in its effect as 
to affect and submerge the hypothetical Jurassic bridge of the Windward 
region. If so, the Windward barrier to the eastward was crossed by 
Atlantic waters, which probably came in across the north side of South- 
ern Florida, which until very recent time has been West Indian in its 
relations. The absence of any known Lower Cretaceous fossils in the 
Antillean region suggests that a large land area may have existed during 
this epoch, composed of south Floridian, Bahaman, Antillean, and Wind- 
ward lands. 
The faunas of Trinidad and the northern regions of South America 
show that in Lower Cretaceous time the Atlantic waters were overcom- 
ing the southern end of the Jurassic Windward bridge, if it ever existed, 
and that the Atlantic littoral faunas were encroaching upon the present 
eastern Caribbean area, which had lost or was losing its connection with 
the Pacific. 
In Middle Cretaceous time there was a considerable movement in the 
northern Gulf region, causing the land to emerge and the shore line to 
recede from southwestern Kansas to east central Texas. We have ab- 
solute record of the extent of this movement in the known migration of 
the line of the Dakota littoral. Inasmuch as all of the sedimental evi- 
dences in the littoral formations of our Coastal Plain are only the margi- 
nal phenomena of oscillations which probably had their greatest amplitude 
to the southward, the effects of this Mid-Cretaceous movement on the 
tropical regions must have been great. It was one of those oscillations 
which, had it occurred in later geologic time, would have been of con- 
siderable importance, but its effects and extent are completely obliterated 
by the grander changes which preceded and followed it. It is probable 
that a Central American land bridge connecting the continents via the 
