78 HAWAIIAN AND OTHER PACIFIC ECHINI. 



pair. This difference is due to the slanting of the sutures upon passing 

 through the test. Dialithocidaris (PI. 23, Panamic Ech.) is more closely allied 

 to Podocidaris than to any other of the Arbaciadai, while the crowding of the 

 poriferous zones at the ambitus allies it more to Arbacia than to such genera 

 as Habrocidaris, Pygraaeocidaris and Podocidaris. 



In Podocidaris sctdpta, in a specimen measuring 11.75 mm. in diameter, 

 there are, beginning at the actinal system, five and four primary ambulacral 

 tubercules. In the interambulacral system the primary tubercules are ar- 

 ranged in transverse rows of three small ones, four somewhat larger, four still 

 larger, five still larger at the ambitus, with a central odd primary in the 

 median line, and the next and last row with only one large tubercule in the 

 outer angle of each interambulacral plate, a much more Arbacian-like ar- 

 rangement than that of the species once associated as Podocidaris. 



The two species of this genus are distinguished from each other as 

 follows : 



Abactinal system, about .60 h. d.; actinostome distinctly pentagonal argentea. 



Abactiual system, .50-.55 li. d. ; actinostome circular scutata. 



Habrocidaris argentea A. Ag. and Cl. 



Habrocidaria argentea A. Agassiz and Clark, 1907. 

 Bull. M. C. Z., L, p. 234. 



Plate 49, figs. 9-U ; 54, figs. 1-3. 



A single specimen of this species was collected by the "Albatross" near 

 French Frigate Shoal. Its diameter is 11.5 mm., that of the actinal system 

 7 mm., of the anal 2 mm., and of the abactinal 4 mm. Unfortunately all the 

 primary radioles are broken and only the basal portions of a few remain at- 

 tached to the test. The radioles, like those of H. scutata (PI. 54, figs. 8 and 9), 

 are triangular in cross section, and the three edges though rounded project 

 conspicuously from the solid axis. The test is silvery, tinged with brown, 

 and the primary radioles were evidently white. This species is closely allied 

 to H. scutata (PI. 54, figs. ^, 5) from which it differs in having a distinctly 

 pentagonal, slightly indented actinal system and larger actinal plates (PI. 54, 

 fig. 1) with comparatively large and prominent poriferous plates, while in H. 

 scutata (Pi. 54, fig. ^) the actinal poriferous pairs of plates are very small, 

 scarcely distinguishable from the great number of small angular plates cover- 

 ing the actinal system. In both species the abactinal system is distinctly 



