f^. 
82 OEIGIN OF THE WEST INDIAN ECHINID FAUNA. 
Of the so-called American genera, all containing most closely allied rejire- 
sentative species, Agassizia, Moira, Meoma, Macropneustes, Arbacia, Encope, 
and Mellita, which probably flourished in the central American seas soon after 
the closing of the Isthmus of Panama, the three Spatangoids date back to 
the Cretaceous, the two Clypeastroids and two Echinida? to the later Ter- 
tiary. We find tlie nearest allies of the Clypeastroids in the Tertiaries of 
Western France and of Eg^-pt; the above-named West Indian Spatangoids 
and Clypeastroids, as well as Coelopleurus and Macropneustes, first disap- 
peared from the Eastern Atlantic. The past history of the ten West In- 
dian genera already found in the Cretaceous, and of the tw^enty-four genera 
descending from the earlier Tertiary, gives us but little assistance in deter- 
mining their probable mode of appearance "in the Caribbean Fauna. 
As far as we can now judge, the separation of the Caribbean and the Gulf 
of Mexico from the Pacific was brought about by the formation of the Florida 
and Yucatan Banks by their elevation above the level of the sea, in addition 
to the raising of the Greater and Lesser Antilles, of the Plateau of Mexico, 
and of the whole of Central America, ending in the complete closing of all 
connection at the Isthmus of Panama. These elevations have been gradually 
taking place from the close of the Cretaceous period to the most recent Post- 
tertiary times, and to the successive changes they have brought about in the 
physical conditions of the Gulf of Mexico and of the Caribbean Sea we 
must ascribe in the main the existino; state of the West Indian Echinid Fauna 
as compared with the Echinid Fauna of other geographical districts. 
•^ It would he most interesting to be able to make a comparison of the deep- 
sea Panamic Fauna with that of the Caribbean, and ascertain if in the con- 
tinental and abyssal regions, at the depths beyond which the effects of light 
and of heat are not prominent factors, we find as marked a difference in the 
/ representative species as in those of the littoral Fauna. 
-^ The West Indian Echinid Fauna comprises more than a quarter of all the 
known species of Echini, and if we take what we might call the tropical At- 
lantic Fauna it includes a little over one third of nil the known species. The 
known species of the Indo-Pacific realm, wliicli we might call the tropical 
Indo-Pacific Fauna, are somewhat more numerous ; so that we have more 
than two thirds of all the known species of Echini belonging to this great 
tropical oceanic belt, the northern and southern limits of which extend some- 
what into the temperate regions. This leaves less than one third of the 
known Echini as representatives of the other faunal districts. There are 
