ORIGIN OF THE WEST INDIAN ECHINID FAI V \. 



only al presenl iliin \ -four genera characteristic of the [ndo-Pacific nol found 

 in the Atlantic, and only eighl genera characteristic of the Atlantic not as 

 yel discovered in the Pacific, while the Atlantic and Pacific have thirty-six 

 genera in common. Of the genera they have in common, 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 genera of Ananchytidae and Pourtalesiae. 



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 Diadematidae, of Ananchytidae, and of Pourtalesiae, derived from the Cre- 

 taceous. The Echinometradae genera of the 1'aeilic 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 [ndo-Pacific marine realms began. Before that time the equa- 

 torial currents probably swept nearly uninterruptedly 'round the globe, and 

 maintained across the [ndo-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 Smith America. 

 If the physical causes we now see at work have, as they became changed, 

 also moiliiied 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 greal equatorial bell 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 hack to the Tertiary period. 



How far it is possible for us directi) 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 hack from the Triassic and Jurassic periods 

 the genera which have appeared in succession, and how far this is prac- 

 ticable I have attempted to .-how on a former occasion. 1 l would also 



• We should aim bear in mind that, of : tic of the Atlantic alone, we find 



in the Indo-P 

 t Pal ind Embryol 



Ajnerii 



