SPEROSOMA GIGANTEUM. 197 



Hansen, which Mortensen found in the spines of Arceosoma gracile. They 

 produce a swelling in the shaft of the spine, with a small opening at the 

 distal end, giving water access to the cavity in which the animal lives. Not 

 rarely there are two of those parasites in the shaft of a single spine. 



The " Albatross " took the species at the following stations : 



Station 4957. Between Kagoshima and Kobe, Japan ; 32° 36' N., 132° 

 23' E. Bott. temp. 39.8°. 437 fathoms. Gn.-bn. m., fine. gy. s., for. 



Station 5079. Off Omai Saki, Japan; 34° 15' N., 138° E. Bott. temp. 

 39.1°. 475-505 fathoms. P. 



Station 5080. Off Omai Saki, Japan ; 34° 10' 30" N., 138° 40' E. Bott. 

 temp. 38.7°. 505 fathoms. Fne. gy. s., glob. 



Bathymetrical range, 437-505 fathoms. Extremes of temperature, 

 39.8°-38.7°. 



Seven specimens. 



Sperosoma giganteum. A. Ag. and CI. 



Sperosoma giganteum. A. Agassiz and Clark, 1907. Bull. M. C. Z., LI, p. 120. 

 PI. 64, figs. 9-12 ; 65, figs. 2-5 ; 83-86. 



This remarkable sea urchin measures nearly 320 mm. in its greatest di- 

 ameter. The color is deep purple, almost black. The ambulacral area is 

 extraordinarily wide, for on the abactinal surface just above the ambitus it 

 measures over 110 mm., while the interambulacrum is a little over 80 mm. 

 (PI. 83). The outer and inner columns in each half of each ambulacrum are 

 made up of remarkably long low plates, which just above the ambitus are 25 

 mm. long and only 5 mm. high (Pis. 83, 86, fig. 2). There are no primary 

 tubercles above the ambitus, but the whole abactinal surface is rather 

 closely covered with slender secondaries and miliaries (PI. 83). On the 

 actinal surface (Pis. 84, 86, fig. 1) primary spines are fairly numerous but 

 irregularly placed, showing no regular arrangement. Many ambulacral 

 plates have two, and many interambulacral plates four, spines. The areolae 

 are small (PI. 86, fig. 1), the diameter usually less than half the height of 

 the plate. The primary spines of the actinal surface are nearly all broken 

 off ; the remaining ones are seldom 25 mm. long, and terminate in a 

 conspicuous white hoof (PI. 84). 



Both the actinal and abactinal systems of this species (PI. 85, figs. 2, 1) 

 differ greatly from the figures given by Koehler of S. G-rimaldii, as well as 



