178 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



through the outer pore of the pair. Unless otherwise noted, the colors 

 given are those of dried Museum specimens. 



In using this, and all other keys given, it should he constantly borne 

 in mind that the younger the individual, the less will it show generic 

 and specific characters ; in proportions, number of coronal plates, and of 

 secondary and miliary spines, arrangement of the abactinal system, form 

 of the primary spines, and color, the young are often quite different 

 from the adults. They can only be identified with certainty on com- 

 parison with other specimens, old, young, and intermediate, and usually, 

 for very young specimens, it is necessary to know the place and means 

 of collection. On the other hand, unusually large specimens often have 

 the abactinal system and actinostome relatively smaller than in speci- 

 mens of more moderate size. Variations of five per cent or more, on 

 either side of any mean given, may therefore be expected. The keys 

 are all based on supposedly normal, mature specimens, the age being 

 estimated by the presence and size of the genital openings, the appearance 

 of the primary spines and abactinal system, and to some extent by the 

 size. Although the radial plates of the abactinal system are not con- 

 nected with any sort of light-detecting or visual organs, they have been 

 so generally called " ocular " (pcellar in German and ocellaires in French) 

 plates that the name is here retained, as preferable to the alternative 

 term "radial," which Duncan uses, but which is not really quite so 

 distinctive. 



Key to the Genera. 



Genera marked with an * have no living representatives. 



Pores horizontal or nearly so, distant (space between the two of a pair 

 evidently exceeding diameter of a pore) ; surface of interval flat, or 

 with a groove connecting pores, never elevated. (Individuals in 

 which this feature is obscure are characterized by stout or more 

 or less thorny spines, 1.5-2 5 h. d. [if less than 1.5 h. d., coronal plates 

 very few, 5 or 6], and unsunken and, even actinally, quite distinct 

 areolae.) Recent species exclusively Indo-Pacific. 

 With pores in 4 more or less regular vertical series in each poriferous 

 zone. 

 With 4 vertical series of coronal plates in each interradius from ac- 

 tinostome to ambitus *Tetracidaris 



With only 2 series of coronal plates in each interradius . . . *Diplocidans 

 With pores in only 2 vertical series in each poriferous zone. 

 Ambulacral and interambulacral plates with more or less numerous, 

 nearly circular pits, irregularly scattered *Temnocidaris 



