254 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The very strongly projecting side arm plates carry seven spines 

 and often even eight at the base of the arms ; these spines are slender, 

 tapering and pointed, and quite transparent. The two dorsal spines 

 are very narrow and their length exceeds two and a half segments; 

 they are provided with small, well-spaced, sharp teeth. The other 

 lateral spines are thicker and shorter, provided with stronger and 

 more closely placed denticulations. The length of these spines in- 

 creases from the second, which is shorter than the segment, to the 

 sixth, which usually almost or quite equals two segments. The first 

 ventral spine is extremely small, very short and slender, and becomes 

 converted into a hook a few segments beyond the disk. 



The tentacle scale is small and oval. 



The dorsal surface of the disk is of a rather light pinkish-gray 

 color; the arms are more red. I find no special ornamentation ex- 

 cept upon the radial shields, which have a purple line along their 

 borders. Along the median dorsal line of the arms runs a white 

 band bordered on either side by a purple line ; there is another wavy 

 and irregular purple line near the lateral borders of the upper arm 

 plates. The ventral surface of the arms shows a broad median 

 white band bounded by two somewhat irregular and wavy purple 

 lines ; the rest of the surface of these plates is pink with a few purple 

 spots. The arm spines and the spines of the dorsal surface of the 

 disk are faintly pink and always very transparent. The ventral sur- 

 face of the disk is dark brown. 



Affinities and distinctive features. — Ophiothrix pavida may be 

 compared with the species of the O. suensoni group, although the 

 arms spines do not have the great length usually found in that group. 

 It recalls especially O. proteus Kcehler, O. picteti Loriol, and 

 O. comata Miiller and Troschel. It differs from O. proteus in hav- 

 ing the radial shields smaller, and sometimes carrying large spines, 

 while the interradial spaces are very much broader, the upper arm 

 plates broader, the mouth shields longer and less flattened, and the 

 arm spines less developed. It differs from O. comata in having the 

 upper arm plates longer than broad, the under arm plates with a 

 slightly rounded distal border, the spines on the dorsal surface 

 of the disk numerous and less elongated, and also passing onto the 

 radial shields, and the mouth shields longer and less broadened. It 

 differs from O. picteti in the presence of club spines among the spines 

 of the dorsal surface of the disk, in having the radial shields fur- 

 nished with spines, the arm spines shorter (Loriol says that in O. 

 picteti the length of these spines reaches that of five or six segments) , 

 and the upper arm plates a little broader than long. 



Its affinities with O. picteti seem to me to be rather pronounced, 

 and as that species is little known, I have thought it well to give here 



