SPEROSOMA OBSCURUM. 199 



The triphyllous pedicellarioe are not peculiar, save that the valves (PI. 

 64, fig. 12) are rather wide, with a fairly well developed and apparently 

 imperforate cover-plate ; they measure about half a millimeter in length and 

 more than half that in width at the tip. 



The ophicephalous pedicellarise (PI. 65, figs. 1, 2) are very common on the 

 abactinal surface, just above the ambitus, but become less common as we 

 pass toward either pole, and are practically wanting at a distance of 75 

 mm. from the ambitus. The stalks are three or four times as long as the 

 heads. The valves (PI. 65, fig. 3) are .60-. 70 mm. in length and have the 

 form usual in the Echinothuridae. 



The sphseridia and calcareous spicules (perforated plates) in the tube-feet 

 show no noteworthy features. 



The single specimen of this species was taken by the " Albatross " at 

 the following station : 



Station 5082. Off Omai Saki Light, Honshu Island, Japan ; 34° 5' N., 

 137° 59' E. Bott. temp. 37.7°. 662 fathoms. Gn. m., fne. s., glob. 



Sperosoma obscurum A. Ag. and ci. 



Sperosoma obscurum A. Agassiz and Clark, 1907. Bull. M. C. Z., L., p. 239. 



Plates 62, fig. 4; 63, fig. l; 65, figs. 4-U; 87-89. 



A large number of specimens of Sperosoma were collected among the 

 Hawaiian Islands, which could not be referred to any previously known 

 species of the genus. In a specimen measuring 169 mm. in diameter there 

 are 19 interambulacral plates (PI. 87) between the ambitus and the abactinal 

 system in each column, and 25 ambulacral plates. At the ambitus the 

 interambulacral area measures 55 mm. across and the ambulacral 50. On 

 the abactinal surface there are comparatively few primary tubercles, forming 

 irregular vertical rows on each side of the interambulacral area. They 

 have large scrobicular circles (PI. 87), and carry comparatively stout 

 spines. The rest of the abactinal surface of the test is covered with distant 

 small, sharp, and slender secondary and miliary spines (PI. 89, fig. 4). 



On the actinal side there are 18 ambulacral and 13 interambulacral 

 plates between the actinal system and the ambitus. Each of the central 

 ambulacral plates near the ambitus carries one large primary tubercle (Pis. 

 88, 89, fig. 8), and in the interambulacral area (Pis. 88, 89, fig. 4) there is 

 one at each extremity of the plate. 



