ECHINOMETRID^:. 367 



most commonly bihamate but in Echinometra they are often triradiate. In 

 Parasalenia, they are very peculiar in having two little projections on the con- 

 cave side, quite unlike anything seen in other Echini. 



THE GENERA AND SPECIES OF ECHINOMETRID^E. 



Generic distinctions in this family are quite sharp and it is easy to distinguish 

 the five which seem valid. Specific distinctions are on the contrary remarkably 

 ill-defined and except in Podophora, it is a matter of difficulty to decide just how 

 many species ought to be recognized. Doderlein's attempt to distinguish Echino- 

 metra oblonga as the type of a new genus seems peculiarly unfortunate, not merely 

 because the character is microscopic but because it is very variable and is not 

 distinctive. Triradiate spicules occur in the pedicels of Echinometra picta, 

 Van Brunti, and especially viridis, as well as in those of oblonga. It is true that 

 some specimens of oblonga have a much greater development of such spicules 

 than are ever seen in any other species, but there is great individual diversity; 

 I have never seen a specimen with nearly as many as those photographed by 

 Doderlein appear to have possessed, while on the other hand, some specimens 

 have very few. Far from considering oblonga as worthy of generic rank, de 

 Meijere's view that it is simply an extreme form of mathcei seems much more 

 probable, for a considerable number of specimens have been seen which could be 

 assigned to one species or the other only quite arbitrarily. 



The characters which are of most service in distinguishing the genera of this 

 family, are the position of the long axis, the structure of the ambulacra, the 

 covering of the periproct, and the character of the spines. 



The following table will show how these are grouped in the different genera 

 here recognized: 



Long axis through IIIb-5b; ambulacral plates of 3 elements; periproct covered 



by 4 subequal plates; spines not peculiar Parasalenia. 



Long axis to the left of IIIb-5b; ambulacral plates of more than 3 elements; peri- 

 proct with numerous plates. 



Long axis through 3-1 ; ambulacral plates of 4-8 elements; spines not peculiar Echinometra. 

 Long axis through IVb-lb or 4a-IIa; ambulacral plates of 9-19 elements; spines 



extraordinarily modified. 

 Primary spines very short, thick and flat-topped, forming a smooth abacti- 



nal pavement. 

 Primary tubercles very large, at least in mid-zone, 3-6 in an irregular 



horizontal row on each interambulacral plate Podophora. 



Primary tubercles moderate, in mid-zone 10-12 in two horizontal 



series on each interambulacral plate Colobocentrotus. 



Primary spines remarkably long, stout, heavy Heterocentrotus. 



