THE DEEP-SEA FAUNA. 159 



torial belt o£ the Atlantic and Indo-Pacific region. Others have 

 an Atlantic and Pacific range ; others again only a more or less 

 limited range in the Caribbean, Indian Ocean, and East Indian 

 Archipelago, and with nearly the same distribution as in the 

 tertiary, having died out from the eastern North Atlantic region 

 where they once flourished. A few genera are strictly tropical 

 American, occurring both on the Pacific and Atlantic sides of 

 the continent ; but they formerly had a much wider geograph- 

 ical distribution, having once lived in the tertiary of Egypt and 

 Australia. 



Some of the clypeastroids date back to the later tertiary, and 

 they are eminently tropical American, occurring on both sides 

 of the continent. 



This leaves a number of genera belonging to the families of 

 the Diadematidse, Ananchytidse, and Pourtalesise, with an ex- 

 tended geographical range in the tropical belt of both the 

 Atlantic and the Pacific oceans, no representatives of which 

 have yet been found fossil. The nearest allies of the Diade- 

 matidae date from the cretaceous, and the others are to-day the 

 representatives of the types of old-fashioned spatangoids which 

 characterized the cretaceous seas. 



This analysis shows that the echinid fauna of the West In- 

 dian seas of to-day is made up, — (1) of five Jurassic genera; 

 (2) of ten genera which go back to the cretaceous period ; (3) 

 of twenty-four genera dating from the earlier tertiary period ; 



(4) of only four genera characteristic of the later tertiaries ; 



(5) of seven genera which we may look upon as the repre- 

 sentatives of the Ananchytidse and Infulasteridse, and of the 

 Pseudodiadematidse, of the cretaceous period. In all these 

 old-fashioned genera there are species having a cosmopolitan 

 range. 



Of the so-called American genera, containing all most closely 

 allied representative species (Agassizia, Moira, Meoma, Macro- 

 pneustes, Encope, Mellita, Arbacia) which probably flourished 

 in the Central American seas soon after the closing- of the 

 Isthmus of Panama, the three spatangoids date back to the cre- 

 taceous, the two clypeastroids and two Echinidse to the later 

 tertiary. The nearest allies of the clypeastroids are found in 



