160 
the tertiaries of Western France and of Egypt; the above-named 
West Indian spatangoids and clypeastroids, as well as Coelo- 
pleurus and Macropneustes, disappeared first from the Eastern 
Atlantic. The past history of the ten West Indian genera al- 
ready found in the cretaceous, and of the twenty-four genera 
descending from the earlier tertiary, gives us but slight assist- 
ance in determining the mode of their origin in the Caribbean 
fauna. 
It would be most interesting to be able to make a compari- 
son of the deep-sea Panamic fauna with that of the Caribbean, 
and to ascertain if, in the continental and abyssal regions, at 
the depths beyond which the effects of motion, of light, and 
of heat cease to be prominent factors, there is as marked a dif- 
ference in the representative species as in those of the littoral 
fauna. 
. Soon after the end of the cretaceous period the specialization 
of the Atlantic and Indo-Pacific marine realms began. Before 
that time the equatorial currents swept nearly uninterruptedly 
round the globe, and maintained across the Indo-Pacific and 
Atlantic the conditions which existed in the Western Atlantic 
before the equatorial currents became deflected by the West 
India Islands and the northern extremity of South America. 
If the physical causes we now see at work have, as they have 
become altered, also modified the fauna of the equatorial belt 
district then existing, we should naturally expect to notice after 
along period of time the changes thus brought about. We 
are perhaps justified in ascribing to the subdivision of this 
equatorial belt, into an Indo-Pacific and an Atlantic district, 
the marked changes perceptible in the character of the fauna 
as regards the genera which date back to the late cretaceous, 
and the changes still more marked in the genera which date 
from the tertiary period. 
How far it is possible for us directly to follow these modifica- 
tions, and to trace the transition of the older fauna into the 
characteristic West Indian fauna of to-day, is another question. 
It involves the necessity of tracing back from the triassie and 
jurassic periods the genera which have appeared in succes- 
sion ; how far this is practicable I have attempted to show on 
THREE CRUISES OF THE “ BLAKE.” 
