12 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 
species of this genus in having the primary ambulacral plates of 
the aboral side undivided. The pedicellariae recall those of Spero- 
soma grimaldii Koehler, to which the present species has otherwise 
apparently no nearer relation. 
I give here figures of the rather large spicules of the tubefeet 
(fig. 2), these having been inadvertently omitted from the Monograph. 
SPEROSOMA TRISTICHUM Mortensen 
Sperosoma tristichum MortTENSEN, Vid. Medd. Dansk Naturh. Foren., vol. 98, 
p. 163, 1934; Monograph of the Echinoidea, vol. 2, p. 197, pl. 12, figs. 2, 3; 
pl. 77, figs. 18-22, 1935. 
Locality—Station 5585; in the vicinity of Sibuko Bay, Borneo; 
Sipadan Island (M.) bearing S. 89° W., 12 miles distant (lat. 
4°07’00’’ N., long. 118°49’54’’ E.); 870 meters; bottom temperature 
5.06° C.; gray mud; September 28, 1909. One specimen. 
Remarks.—Al\so of this species a full description, with figures, is 
given in my Monograph, to which the reader is referred. The speci- 
men has a diameter of 95 mm. 
This species differs markedly from the other known species of 
Sperosoma in the arrangement of the pores on the aboral side in arcs 
of three, instead of the usual arrangement in more or less regular 
quincunx. The pedicellariae are not very characteristic. 
SPEROSOMA species (juv.) 
FIGURE 3 
Localities —Station 5332; in Mindoro Strait; Apo Light bearing 
S. 66° W., 18.2 miles distant (lat. 12°47’15’’ N., long. 120°41’00” E.) ; 
1,362 meters; bottom temperature 3.44° C.; green mud; December 12, 
1909. One specimen. 
Station 5608; Gulf of Tomini, Celebes; Binang Unang peak bear- 
ing S. 87° E., 19 miles distant (lat. 0°08’00” S., long. 121°19’00’’ E.) ; 
1,991 meters; bottom temperature 2.39° C.; gray mud; November 18, 
1909. One specimen. 
Remarks.—The specimen from station 5608 is 25 mm. in diameter, 
and that from station 5332 is 23 mm. in diameter. The number of 
plates in the specimen 25 mm. in diameter (station 5608) is 5-6 
ambulacral plates on the oral, 11-12 on the aboral side, and the same 
numbers respectively in the interambulacra. In the specimen 23 mm. 
in diameter (station 5332) there are 5-6 ambulacral plates on the 
oral, 7-8 on the aboral side, and the same numbers respectively in the 
interambulacra. Both specimens have only three plates in a series on 
the peristome. 
The tridentate pedicellariae are characteristic in having a distinct 
are under the articular surface (fig. 3, @), a character otherwise known 
only in Sperosoma durum Déderlein. In the smaller specimen there 
