446 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Marginal mouth spines, 11 or 12, the inner 3 of each plate pointed, 

 long, the innermost as long as the base line of entire series ; the outer 

 8 or 9 much slenderer and shorter and decreasing in length toward the 

 outer end of margin. All are united b}^ membrane. Suboral spines 

 about 10 (6 in small specimen), in an irregular series parallel to 

 median suture. In the type they are smaller than the marginal 

 spines, but in the small specimen the inner 1 or 2 are about as long 

 as the fourth or fifth marginal spines. Along the median suture 

 margin is a row of low papillae without a calcareous core. 



Madreporic body, 4 mm. in diameter, prominent, surrounded by 

 about 6 pseudopaxillae, and situated its own diameter adcentral to 

 the middle of r. 



Type.—Qd^t. No. 32647, U.S.N.M. 



Type-locality. — Station 5654, Gulf of Boni, Celebes, 805 fathoms; 

 bottom not recorded, bottom temperature 38.3° F. 



Distribution. — Known only from Gulf of Boni and Molucca 

 Passage. 



Specimens examined. — The type and a specimen from station 

 5619, off Mareh Island, Molucca Passage, 435 fathoms, fine gray 

 sand and mud. 



Remarks. — This species is allied to S. paxillatus Sladen, but proba- 

 bly not very closely, while it also shows some resemblance to S. 

 horealis Fisher, though it is not related to this form, unless remotely. 

 The following differences separate tropicus from paxillatus: In 

 tropicus the paxillae have fewer, relatively coarser spinelets; mar- 

 ginal plates lower, with conspicuously heavier spinelets at the inner 

 or lower end than at the upper or outer, and distal marginals occu- 

 pied almost entirely by a relatively few large spinelets (somewhat as 

 in S. horealis) ; in ,6'. paxillatus the inner marginal spinelets are only 

 a little larger than the outer, and the armature of the distal margin- 

 als does not differ materially from that of the proximal; in tropicus 

 the superomarginals are lower and less conspicuous, the furrow 

 spines more numerous (5 or 6 instead of 3 or 4) and in large speci- 

 mens the subambulacral spines are decidedly less prominent (sub- 

 equal to furrow spines). Tropicus differs from horealis in having 

 less prominent marginal plates and much more numerous and smaller 

 marginal spines, distinguishable superomarginal plates, more numer- 

 ous and longer furrow spines and more numerous and longer sub- 

 ambulacral spines. S. torulatus Sladen from north of the Kermadec 

 Islands has less prominent marginals actinally situated, and with 

 subequal spinelets; the furrow spines are shorter, the subambulacral 

 spines fewer and in a straight series; the actinal interradial areas 

 smaller, and the abactinal paxillar spinelets slightly more numerous. 

 8. regularis Sladen has larger and more widely spaced paxillae, 

 more prominent marginal plates, and the subambulacral spines form 



