82 OEIGIN OF THE WEST INDIAN ECHINI!) FAUNA. 



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

 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 Echinidae to the later Ter- 

 tiary. We find the nearest allies of the Clypeastroids in the Tertiaries of 

 Western France and of Egypt ; 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 twenty-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 t he 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 existing state of the West Indian Echinid Fauna 

 as compared with the Echinid Fauna of other geographical districts. 



It would be most interesting to lie 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 

 ami of heal arc 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 all the known specie.-. The 

 known species of the Indo-Pacific realm, which 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 



