426 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



I find in the Albatross collection one specimen (station 5429) with 

 the same dimensions, in which the majority of the arm segments 

 have only two spines; a few of them already have three, but this 

 figure is rare and it only appears, fortuitously, at some distance from 

 the disk. It is therefore quite probable that the number of two arm 

 spines is correlated with the youth of the subject and that the third 

 spine appears later. All the other characters agree so well that it 

 seems to me impossible to separate the Albatross specimens from my 

 O. molesta. I give a figure of the ventral surface of the young 

 example of which I have just spoken (pi. 79, fig. 5), and this may 

 be compared with the figure which I have previously given of O. 

 molesta ('04, p. 6, fig. 4) ; a comparison of the photographs which I 

 give here, some of a specimen in which the disk is 10.5 mm. in diam- 

 eter (pi. 78, figs. 3, 4) , and the other (pi. 79, fig. 10) of a specimen 

 in which the disk is 8 mm. in diameter, with the corresponding figure 

 of the type specimen ('04, pi. 6, fig. 3), shows well the identity of 

 these different forms. 



It seems to me worth while to describe this species again from adult 

 examples. 



Description. — The diameter of the disk varies between 9 mm. and 

 12 mm. In a specimen from station 5114 which I have taken as the 

 type (pi. 78, figs. 3, 4) the diameter of the disk is. 10.5 mm. and the 

 arms are 25 mm. long ; the habitus of the animal is very robust. The 

 disk is rounded and rather thick; the arms are short, very broad at 

 the base, and taper rapidly to the tip. 



The two surfaces of the disk are plane and parallel, and they pass 

 into each other over a rounded border ; the outline of the disk is also 

 rounded. The dorsal surface is covered with unequal and slightly 

 imbricated plates, among which a primary rosette of large plates 

 separated from each other by a single row of very much smaller 

 plates may be distinguished. The rounded dorso-central is larger 

 than the radial plates, which are slightly broadened transversely. 

 Among the other plates there may be distinguished an interradial 

 row, including usually three rather large tandem plates, and, in the 

 interradii, two plates of which the outer penetrates between the two 

 radial shields of each pair. These plates are more or less polygonal. 

 The other plates are small, unequal, more or less polygonal in out- 

 line, and, in the interradii, there are generally two or three rows on 

 each side of the median line. The radial shields, which are rather 

 small, are triangular, longer than broad, with a sharp proximal angle 

 and a slightly convex distal border ; their length is equal to a third 

 of the radius of the disk. The two shields of each pair are divergent 

 inwardly, and they are separated throughout their whole length, 

 proximally by two or three plates, and distally by a single plate. 



