240 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



these two species. In his type the diameter of the disk measured 

 8 mm., and the arms were 65 mm. long. 



My specimen agrees exactly with H. L. Clark's description and 

 photographs. The dorsal surface of the disk is light red, while the 

 arms have a bluish red coloration. The white marblings of the ra- 

 dial shields are little marked, though quite recognizable. The white 

 longitudinal line which runs all along the dorsal surface of the arms 

 is very well marked, and it is bordered on either side by a .red line ; 

 some white spots appear beyond it, and at some distance from the 

 median line. These successive spots on the side arm plates form an 

 irregular and interrupted line which is situated toward the dividing 

 line between the dorsal and side arm plates; these last each show 

 another white spot. The ventral surface of the disk and of the arms 

 show the coloration described by H. L. Clark; the arm spines are 

 glassy and slightly bluish. 



The only difference that I notice between my specimen and H. L. 

 Clark's type is that the spines of the dorsal surface of the disk ap- 

 pear to me a little thicker and less pointed than in the latter (pi. 

 99, fig. 3a) ; this slight difference may, however, be correlated with 

 the somewhat smaller size. 



Ophiothrix hybrida may be compared with 0. tennis Brock and 

 O. virgata Lyman, but it differs from both of these species by its col- 

 oration ; furthermore, it differs from 0. tennis in the form of the up- 

 per and under arm plates, in the fourth dorsal spine, which is not 

 broadened, and in the radial shields which are not in contact in each 

 pair ; it also differs from O. virgata in the smaller radial shields, in 

 the upper arm plates, etc. 



The tentacle scale is large with one or two short points on its free 

 border (fig. 3<?). The hook-like form which the first ventral spine as- 

 sumes at some distance from the disk is interesting (fig. 3d) ; it is, in 

 fact, intermediate between that usually seen in the genus Ophiothrix 

 and that which I have recorded in O. longipeda, O. puncto- 

 linibata, 0. propinqua, etc. The hook is elongated and it ends 

 in a strong recurved point behind which there is another, weaker; 

 following this are four smaller subequal points, which are straight 

 and parallel to each other. These mixed characters offer an argu- 

 ment in favor of H. L. Clark's hypothesis that O. hybrbda is a hybrid 

 between O. longipdfla and 0. trilineata. 



The exact habitat of H. L. Clark's unique specimen is not certain ; 

 it was purchased from the Museum Godeffroy and was labeled " Fiji 

 or Samoa." 



