ASPIDODIADEMATID.E. 59 



while in the other Diadematidre the oculars, though in contact with the 

 anal system, yet are small compared to the large triangular genital plates of 

 the family. 



The auricles arc most irregularly developed. They are either wanting 

 or mere projections, slightly raised. 



Since the publication, in 1898, of the Preliminary Report of the Echini 

 of the 1891 " Albatross " Expedition, my attention has been called by a 

 synonym in Duncan's Revision of the Genera of the Echinoidea, p. 56, to the 

 fact that Pomel in 1883 separated Aspidodiadema microtuberculatum from 

 A. tonsum, and suggested a new generic name, Plesiodiadema, differing 

 from Aspidodiadema in having, according to Pomel, a double row of small 

 ambulacral tubercles, while A. tonsum has only granules, as in Cidaris. 

 Unfortunately the reverse is the case, and I may be pardoned in rejecting 

 a name which is evidently based upon a misunderstanding or a gross error 

 in quotation, as has also been noticed by Lambert, 1. c. p. 14. 



When Aspidodiadema antiUarum was described, there were included with 

 it in the genus Aspidodiadema other species not having the peculiar 

 Hemicidaris arrangement of the actinal ambulacral primary tubercles. 

 Their absence was considered as dne to the small size of the specimens 

 then available. The material of the genus collected in the Panamic realm 

 shows that A. antiUarum of the " Blake" as well as A. microtuberculatum of 

 the " Challenger " collection belong to the genus Dermatodiadema, as here 

 limited, in which the actinal secondary tubercles are small, hardly larger 

 than the iniliaries. 



The genus Dermatodiadema in the Pacific seems to be characteristic of 

 deep water. The least depth at which either species has been dredged 

 is over 900 fathoms, and the deepest point, north of Malpelo Island, 

 is nearly 1800 fathoms. A. antiUarum has a range of about 400 to nearly 

 1600 fathoms, and the other species of Dermatodiadema range from 400 to 

 over 2200 fathoms, while Aspidodiadema as now restricted 1 has a range 

 starting from much shallower waters, — about 100 fathoms. 



1 A. Jacobyi, A. tonsum. 



