ORIGIN OF THE WEST INDIAN ECHINID FAUNA. 83 
only at present thii-ty-lbur genera characteristic of tlie Indo-Pacific not found 
in the Atlantic, and only eight genera characteristic of the Atlantic not as 
yet discovered in the Pacific,* while the Atlantic and Pacific have thirty-six 
genera in common. Of the genera they have in connnon, four date back to 
the Jura, seven to the Cretaceous, sixteen to the Tertiary ; the others belong 
to the Diadematidae, which have their nearest allies in the Cretaceous, as 
well as the five recent o;enera of Ananchvtidse and Pourtalesioe. 
Of the genera special to the Indo-Pacific, two date back to the Jura, as 
many to the Cretaceous, twenty-one to the Tertiary ; there are left the genera 
of Diadematidce, of Ananchytidaj, and of Pourtalesiae, derived from the Cre- 
taceous. The Echinometradge genera of the Pacific have no fossil repre- 
sentatives. Of the special Atlantic genera, two are Jurassic, two Cretaceous, 
one Tertiary ; the other has no fossil representative. 
Soon after the end of the Cretaceous period the specialization of the great 
Atlantic and Indo-Pacific marine realms began. Before that time the equa- 
torial currents probably swept nearly uninterruptedly round the globe, and 
maintained across the Indo-Pacific and Atlantic nearly the conditions exist- 
ing 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 became changed, 
also modified the Fauna of the then existing equatorial belt district, we should 
naturally expect to notice after a long period of time the changes they 
brought about. We are probably justified in ascribing to the subdivision 
of this great equatorial belt into an Indo-Pacific and an Atlantic district the 
marked changes we can trace in the character of the Fauna as affecting the 
genera which date back to the late Cretaceous, and which become still more 
marked if we trace them in the genera dating back to the Tertiary period. 
How far it is possible for us directly to follow the changes which have 
taken place, and to trace the gradual passage of the older Fauna into the 
characteristic West Indian Fauna of to-day, is another question. This in- 
volves the necessity of tracing back from the Triassic and Jurassic periods 
the genera which have appeared in succession, and how for this is prac- 
ticable I have attempted to show on a former occasion.! I would also 
* We should also bear in mind that, of the eight genera characteristic of the Atlantic alone, we find 
closely allied representative genera in the Indo-Pacific realm. 
t Palccontological and Emhryological Development, Address at the Boston Meeting for 1880 of the 
American Association for the Advancement of Science. 
