HABROCIDAPJS. 



7 



and attempted further to trace out this afBnity in oid and young Arbaciadae 

 by showing the permanence of this plate in Diahthocidaris ^ and in Arbacia 

 stellata? 



Our attention was again called to the subject by Doderlein's interesting 

 discovery ^ of the existence of a well-developed primordial interambulacral 

 plate in Podocidaris prionigera K. kg.,iov^\\\Q,\\ he established the genus Pyg- 

 maeocidaris. He furthermore found that the interambulacral zones consist 

 at the base of three plates much as in Tiarechinns. Pygmseocidaris seems to 

 be slightly more developed than Tiarechinus, however, having beyond the 

 row of three plates, interambulacral zones with two rows of plates. This 

 led us to examine again Podocidaris scidpta and Hcdjrocidarh scutaia^ in 

 connection with the new H. argentea from the Hawaiian Islands, and to our 

 gratification we found that in all the primordial plate is as fully developed 

 as in PygmcBocidaris prionigera. The details of this primitive structure in H. 

 argentea and H. scutata are fully shown on PL 54, figs. l-Jf., 6, 7 of this 

 Memoir. Neither of these species, however, has the third interambulacral 

 plate with a large tubercle immediately above the primordial plate, as 

 shown by Doderlein in figs, e, f, p. 184, of the '' Valdivia" Echini. Yet we 

 can agree fully with Doderlein's view of the affinity of these interesting 

 species of Arbaciadae, as well as of the family, to the Triassic Tiarechinus. 



On re-examination of a speci- 

 I men of Podocidaris scidpta 7 mm. 

 in diameter we find that the pri- 

 mordial plate (figs, a, b) extends 

 abactinally so as to separate the 

 first two pairs of interambulacral 

 plates, slightly encroaching upon the third pair, while in 

 Habrocidaris it sepjirates only the first pair slightly en- 

 croaching upon the second pair. The primordial plate 

 is not cut into two plates as it is figured for Pyg- 

 mgeocidaris by Doderlein, pp. 184, 185, " Valdivia " 

 Echini. Seen from the interior of the test, fig. a, the 

 primordial plate only separates the first pair of inter- 

 ambulacral plates and encroaches upon the second 



1 Panamic Deep Sea Ecliini, Mem. M. C. Z., XXXI, PI. 23, figs. 1, 4, 6 



2 Mem. IM. C. Z., XXXT, PI. 51, figs. 5, 6. 

 8 "Valdivia" Echini, p. 182 (figs. p. 183). 

 < Collected by the " Blake " off Santa Cruz, in 580 fathoms. 



Fig. b. 



