CLARK : THE CIDARIDAE. 215 



bulacral area rather wide, bare, and slightly sunken along sutural line ; ambulacra 

 about .30 of interambulaera ; poriferous zones almost flush with test; ambulacral 

 plates few, 30-32 in largest specimens ; median ambulacral area somewhat wider 

 than a poriferous zone; each ambulacral plate is vertically wide and carries only 

 a single tubercle, except in large specimens, when a second smaller tubercle is 

 present and then vertical suture is obscured ; pores very close together, somewhat 

 oblique. Abactinal system very large, .60-70 h. d., either flat or more or less ele- 

 vated, with few or many tubercles. Actinostome .40-.50 h. d., consequently only 

 .G0-.80 of abactinal system, and notable for small number of plates borne by mem- 

 brane, more or less of which near outer margin is quite bare. Primary spines slen- 

 der, straight, and cylindrical, very finely prickly, white or nearly so, 1.5-3 h. d. ; 

 actinal primaries either coarsely or finely serrate or entire; secondaries and milia- 

 ries alike, cylindrical or club-shaped, blunt and more or less erect, rather scattered. 

 Pedicellariae of only one kind, globiferous, but very variable in size. 



The affinities of this interesting genus are rather obscure, for although the sec- 

 ondary spines resemble those of Aust rocidaris nutrix, it is hard to believe that 

 there is any close relationship to that species. There are no other living species 

 of Cidaridae which approach sufficiently near the three rare species placed here 

 to give us any real clue to their natural position. Although A. milleri has actinal 

 primaries similar to those of Porocidaris, there is little else to ally it with that 

 genus, and the other two species are even more different. The large abactinal 

 system, few ambulacral plates, unsunken poriferous zones, somewhat bare actino- 

 stome, and the primary spines are striking reminders of Salenia. Two of the 

 species are discoveries made by the "Albatross" and are found only in the deep 

 waters of the Pacific Ocean ; although milleri was once taken in 465 fths., most of 

 the specimens are from over 1,600 fths. and/rat/ilis has been taken only at depths 

 exceeding 1,500 fths. The third species was found by the " Belgica " in much 

 shallower water, but in the far Autarctic Ocean. The following key is based on 

 the examination of 116 specimens of the two "Albatross" species. 



Key to the Species. 

 Test moderately high, .55 h. d. and more ; abactinal system elevated, with 

 numerous tubercles (250-300 on a system 13 mm. across) ; ambulacral 



plates about 20, in a specimen 15 mm. h. d milleri 



Test flat, about .50 b. d. ; abactinal system not elevated, with comparatively 

 few tubercles (100-200 on a system 13 mm. across) ; ambulacral plates 

 about 15, in a specimen 15 mm. h. d. 



Color reddish- or yellowish-brown ; arctic fragilis 



Color bay or reddish ; antarctic incerta 



Aporocidaris milleri. 

 Porocidaris Milleri A. Agassiz, 1898, Bull. M. C. Z. 32, 5, p. 74. 

 Aporocidaris Milleri A. Agassiz and Clark, 1907, Haw. Pac. Ech. Cid., p. 37. 



Plate 6, Pan. Deep Sea Ech., A. Agassiz, 1904. 



The test of this species is grayish, sometimes with a purple tinge, or yellowish, 

 and the secondaries are of about the same color or paler. The primaries are 



