158 THREE CRUISES OF THE "' BLAKE." 



by the elevation of the Isthmus of Panama and the Mexican 

 Plateau. To 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 

 Pacific, 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 faunae. 



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, Dorocidaris, 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- 



