OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 259 



The slightly projecting side arm plates carry at the base of the 

 arms eight arm spines. The three first ventral spines are very short, 

 and the length increases from the fourth, which almost equals a seg- 

 ment, to the last, which is as long as two segments. The three first 

 ventral spines have a rugose surface, and the first is transformed into 

 a hook a few segments beyond the disk. From the fourth to the 

 penultimate the spines show in their distal half conical, pointed, and 

 stout denticulations, which become more closely crowded toward the 

 tip ; this tip has almost the same thickness as the rest of the spine. 

 But the last dorsal spine tapers gradually to the tip and only shows 

 slightly developed and well-spaced denticulations. All these spines 

 are glassy and very transparent. 



The tentacle scale is small, oval, and a little longer than broad. 



The general color is a light greenish gray ; the radial shields have 

 a few gray-blue spots, and the dorsal plates of the disk also show a 

 few small and barely evident spots. The upper arm plates are 

 studded with elongated dark-blue spots, which tend to arrange them- 

 selves in three irregular and interrupted longitudinal lines, one 

 median and two lateral. The ventral surface of the disk in the 

 parts covered with plates is light gray, but the naked regions are 

 dark brown ; the ventral surface of the arms is light gray ; the under 

 arm plates show a median band which is lighter than the lateral 

 portions, but the difference in the coloration is scarcely noticeable. 



Affinities and distinctive features. — Ophiothrix prostrata seems to 

 me to be near 0. plana, and the plates of the dorsal surface of the 

 disk have an arrangement very much like that seen in that species, 

 but they are provided with cylindrical club spines and the radial 

 shields are relatively larger than in O. plana; the arm spines are 

 provided with denticulations only in their distal half, though this 

 portion of the spines is not broadened. There is also a rather close 

 similarity to 0. liodisca recently described by H. L. Clark ('15, p. 

 273, pi. 12, fig. 3) from Torres Straits. This species is placed by 

 the author near the genus Ophiotrichoides, but it seems to me clearly 

 to belong to the group of 0. plana; judging from H. L. Clark's 

 photograph the arrangement of the dorsal plates of the disk is 

 identical with that shown in this species ; the arm spines are slightly 

 broadened in their terminal portion, which bears denticulations ; on 

 the other hand, 0. liodisca differs sharply from O. plana in its very 

 long arms. Ophiothrix prostrata is distinguished from O. liodisca 

 by the same characters which differentiate it from O. plana. The 

 form of the arm spines and the arrangement of the dorsal plates 

 of the disk bring O. prostrata near O. foveolata, but it differs from 

 it at once in the presence of small club-spines instead of true spines 

 on the dorsal surface of the disk, in the elongated upper and under 

 arm plates, in its larger mouth shields, and in its coloration. There 



