240 GEOGRAPHICAL DISTRIBUTION OF THE GENERA. 



GEOGRAPHICAL DISTRIBUTION OF THE 



GENERA. 



The geographical distribution of the genera, which has formed the basis 

 for the division of the oceans into the four great realms [PL G) recognized 

 among recent Echini, is given in detail in Plates C, D, E, and F. It has heen 

 found impracticable to arrange them on the maps systematically, on account 

 of the color printing and the economy of space. Those genera, therefore, 

 which have a very similar geographical distribution are brought together, 

 or are so limited as not to interfere with the ready understanding of the 

 limits of the other genera represented. It lias not been found possible 

 to give always the precise limit of the extension of each genus, but as 

 the exact localities of each species are given with the synonymy, this 

 is not a serious defect, and the range of each genus is sketched out in 

 broad outlines. Genera which have nearly the same range, but differ 

 in some minor points, are represented as identical, — a method, of course, 

 not strictly accurate, but sufficiently so to call up in a broad way the 

 districts to which these genera are limited ; as, for example, in PL D 

 the range of the genera Pseudoboletia, Phyllacanthus, Colobocentrotus, etc., 

 which means simply that the extremes of range are so nearly identical that 

 although the localities from which some of the genera are known to occur 

 are more numerous than others, yet if we intercalate between the extremes, 

 instead of adopting the more accurate method of mapping only what is 

 known, we shall arrive at about the same geographical distribution. In order 

 not to introduce too many colors when the range of a genus is nearly co- 

 extensive with that of two or more genera, the different colors characteristic of 

 the separate genera are given for the genus having the widest range : for in- 

 stance, in PL D the range of Echinoneus is made up of its own color in addi- 

 tion to the colors indicating the range of Pseudoboletia, Phyllacanthus, etc. ; 

 Salmacis, Echinodiscus, etc., range over a still more limited district, included 

 not only within the limits of the district over which Echinoneus is found, but 

 also within the limits of the range of Pseudoboletia. In the same way in PI. C, 



