134 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The diameter of the disk in adult specimens varies between 10 mm. 

 and 15 mm., and the arms may reach a length of from 120 mm. to 

 140 mm. 



The disk is thick, more or less strongly excavated in the inter- 

 radial spaces; the radial regions, on the other hand, are projecting. 

 Interradial incisions more or less deep are seen on the border of the 

 disk and continue over the dorsal surface in the form of more or less 

 marked depressions. These depressions, however, are not absolutely 

 constant, and they only occur, so to speak, in the specimen from sta- 

 tion 5123 figured in figure 8 on plate 25. The dorsal surface is cov- 

 ered between the radial shields with numerous irregularly polygonal 

 plates which are small and crowded in the central region becoming 

 larger toward the periphery of the disk. These plates attain their 

 largest dimensions toward the interradial incisions, but without 

 being noticeable for any great size, as is ordinarily the case in the 

 genus Ophioplinthaca. They are provided with formations which 

 vary between simple granulations and rather strong and elongated 

 spines. In the specimen from station 5129 (fig. 7) the central part 

 of the disk shows small, short, conical and closely crowded club 

 spines, terminated by two or three very short spinules, these club 

 spines becoming thicker and less crowded, and passing progressively 

 in(o slightly elongated granules the size of which increases toward 

 the periphery of the disk, and the edges of the radial shields. In the 

 specimen from station 5123, shown in figure 8, the small club spines 

 of the central region preserve their character of club spines in ap- 

 proaching the periphery, simply becoming here a little larger and 

 conical. In the other specimens from station 5123 granules may be 

 observed to pass gradually into small spines. Thus in the specimen 

 represented in figure 4 there occurs a mixture of rather elongated 

 spines and of granules of variable dimensions, the latter smooth, the 

 former terminated by small spinules. In the specimen from station 

 5656, shown in figure 2, there is scarcely anything to be seen except 

 a few spines which are thick and rather long, furnished on their 

 borders with rather strong denticulations. 



In figure 6 on plate 94 1 show the different formations which I have 

 met with on the dorsal surface of the disk — rounded and simply 

 rugose granules, small and short club spines furnished at their ex- 

 tremity with a few short spinules, larger club spines provided with 

 numerous spinules, elongated club spines terminated solely by two 

 or three much developed points, and lastly true spines. 



The radial shields are rather small, elongated, twice as long as 

 broad, oval, or triangular with the angles rounded. They are slightly 

 divergent in their proximal portion, and remain separated for their 

 whole length by an interval which is equal to almost half their width. 



