158 THREE CRUISES OF THY ©“ BLAKE." 
by the elevation of the Isthmus of Panama and the Mexican 
Plateau. То these have been added, especially in the West 
Indian fauna, a number of Atlantic types, which, as long as the 
Gulf of Mexico and the Caribbean were practically a part of the 
Pacifie, perhaps did not find conditions as suitable to their de- 
velopment as those which have existed ever since they became 
merely extensions of the equatorial Atlantic district. 
In short, to the successive changes brought about in the 
physical conditions of the Gulf of Mexico and of the Carib- 
bean, we may ascribe in the main the present state of the West 
Indian fauna, as compared with that of other geographical dis- 
tricts. 
This explanation gives us an apparently good reason for the 
mixed character of the fauna of the West Indian seas, showing 
us at the same time that, however long a period may have 
elapsed since this separation took place, it has not been sufficient 
to effect any radical change in the echinid fauna of the two 
sides of the Isthmus. The principal differences are due to the 
immigration of true Atlantic types into the West Indian faunal 
region during the tertiary and post-tertiary period. But as the 
physical conditions of the sea in the tropical regions of the 
Isthmus are so nearly identical, we could not expect from phy- 
sical causes alone any great differences to arise between the 
Panamic and West Indian faune. 
To ascertain the former distribution of the genera of the 
West Indian echinid fauna, we should trace back as far as pos- 
sible the origin of these genera. We find a few genera, like 
Cidaris, Doracidaris, Porocidaris, and Salenia, which date back 
to the jurassic period, and in the tertiary had as wide a geo- 
graphical distribution as at the present day. 
Hemipedina, as old as the jura, is found fossil in the tertiary 
of North America, and has thus far not been dredged outside 
of the Caribbean fauna. There are no less than ten genera 
which date from the cretaceous period, and some of them had 
during the tertiary as extensive a geographical range as they 
have to-day. 
The genera of the earlier tertiary period characterize largely 
the West Indian fauna. Of these a few extend into the equa- 
