OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 59 



ticularly developed along the proximal border; their tips are rounded. 

 The dorsal spines show only very fine and very closely crowded as- 

 perities ; they are somewhat narrowed, and their tips form an obtuse 

 point. The length increases from the first ventral spine, which is as 

 long as the segment, to the last dorsal, which may be as long as-four 

 segments. The two lateral columns of spines come very close together 

 in the median dorsal line on the first segments, though they are always 

 separated from each other by a small space. 



The tentacle pores of the first pair are very large and appear to be 

 without a tentacle scale, or they may have a single almost rudi- 

 mentary one. The pores of the second pair have a rather large 

 scale, and on one of them the scale is doubled. The following pores 

 have uniformly a single scale of moderate size, which is flattened, 

 rectangular, with a rounded and sometimes even slightly broadened 

 distal border. The surface of these scales is very rugose. 



The disk is very dark, blackish brown, because of the extreme 

 thinness of the plates which cover the underlying tissues ; the arms are 

 white. 



Affinities and distinctive features. Ophiacantha pacata is easily 

 recognized by the form of the spines on the dorsal surface of the 

 disk, which are borne upon extremely thin and translucent plates, by 

 the presence of three mouth papillae, and by the small mouth shields, 

 which are triangular and almost as long as broad. It seems to me 

 to be most closely related to 0. prionota, which H. L. Clark described 

 from a unique specimen from southern Japan in 1,008 fathoms, of 

 which the diameter of the disk was 9 mm. It is distinguished from 

 this species by the stouter and longer spines on the dorsal surface 

 of the disk, broadened at the tip, which bear three spinules; by 

 having the mouth shields slightly more elongated toward the proxi- 

 mal angle ; by the longer adoral plates ; by the more elongated arm 

 spines; by having the upper arm plates broader than long; and by 

 the more elongated and larger under arm plates, which are sepa- 

 rated by a narrow interval. These differences are obviously not cor- 

 related with the difference in the size of the specimens ; as I have just 

 said, in O. prionota the diameter of the disk is 9 mm., and in O. pacata 

 it is 12 mm. ; the difference is not very great. 



OPHIACANTHA PENTAGONA Koehler. 



Plate 21, figs. 1, 5, 6 ; plate 93, fig. 5. 



Ophiacantha pentagona KCEHLER ('96), p. 343, pi. 8, figs. 56, 57; ('99), p. 

 53, pi. 4, figs. 27-29; ('04), p. 110. H. L. CLARK ('11), p. 196; ('15), p. 

 204. MATSITMOTO ('17), p. 116. 



Localities. Albatross station 5116; Balayan Bay and Verde 

 Island Passage; Sombrero Island bearing N. 69 E., 4.63 kilometers 



