DTADEMA SETOSUM. 113 



Abactinal system about .60 of diameter of actino. tome ; second 

 series of tubercles begins on 6th (sometimes 5th or 7th) coronal 

 plate ; ambulacra wider, sometimes ^ of interambulacra. 



Valves of tridentate pedicellarise widened near tip, distinctly 

 curved, not very flat; color usually with more or less of a 



reddish cast Savignyi. 



Valves of tridentate pedicellariae distinctly narrowed near tip, 

 nearly straight, fiat; color usually with an olive cast or 

 nearly uniform black globulosum. 



Diadema setosum Gray. 



Echinometra setosa Leske, 1778. Add. Klein, p. 36. 

 Diadema setosa Gray, 1825. Ann, Phil., X, p. 4. 



We have nothing to add to Mortensen's account of this species under the 

 name saxatile L. The slender tridentate pedicellariae are certainly very 

 characteristic when present, but are unfortunately often wanting, particu- 

 larly in young specimens. Occasionally tridentate pedicellariae of large size 

 are met with in which the valves are much wider than usual and approach 

 the form of those of paucispinum, but as a rule a single glance with a magni- 

 fying glass at one of the large tridentate pedicellariae of a Diadema is suffi- 

 cient to determine whether it is setosum or not. 



This species was taken by the " Albatross " on the reef of Neiafu, Vavau, 

 Tonga Islands. Dec. 5, 1899. One specimen. 



Diadema mexicanum A. Ag. 



Diadema mexicanum A. Agassiz, 1863. Bull. M. C. Z., I, p. 19. 



We have but little to add to Mortensen's description of the pedicellariae 

 of this species, except as regards the large tridentate. The " inward fold " 

 at the tip of the valve which he describes and figures we find to be rather 

 rare in our specimens, but on the other hand there are some details of struc- 

 ture by which these pedicellarise can be distinguished from those of antil- 

 larimi. The valves are generally quite distinctly curved, wide near the tip 

 and somewhat compressed above the basal part, so that the end of the 

 apophysis is Y-shaped, while in antillarum the valves are straigiiter and 

 flatter and the end of the apophysis is T-shaped. The differences are so 

 slight that we are not inclined to lay much stress upon them, and xQt we 

 find they are fairly constant. The pedicels often contain larger and more 



