THE DEEP-SEA FAUNA. 159 
torial belt of 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 
Ameriean, occurring both on the Pacific and Atlantie 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 Diadematide, Ananchytide, and Pourtalesiz, 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- 
matide 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 Ananchytidæ and Infulasteride, and of the 
Pseudodiadematidz, 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 baok to the cre- 
taceous, the two clypeastroids and two Kehinide to the later 
tertiary. The nearest allies of the clypeastroids are found in 
