150 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



elongates into a little narrow stalk quite comparable to that which 

 H. L. Clark has described in 0. polyploca. 



In their dimensions relative to the diameter of the disk the radial 

 shields show certain variations. In a specimen from station 5187 

 (pi. 28, fig. 4) these shields are much elongated and rather narrow; 

 in another from the same station (fig. 6) they are a little shorter and 

 relatively broader; in the specimens from stations 5167 (figs. 1, 3), 

 5540 (pi. 29, fig. 1), and 5541 (pi. 29, fig. 5) they are still shorter; in 

 a specimen from station 5167 (pi. 28, fig. 3) they are, on the other 

 hand, relatively large. As for the armature of the dorsal surface of 

 the disk, it consists sometimes of small, short club spines terminated 

 by a crown of spinules, as may be seen in the specimen from station 

 5167 (pi. 28, fig. 3) ; these club spines may become very much 

 thicker and larger with a crown of very strong spinules, a condition 

 seen in the individual from station 5275 shown in figure 7 on plate 

 27, in which the plates of the dorsal surface of the disk are them- 

 selves very much larger. The club spines may elongate, at the same 

 time remaining slender, and thus constitute a fairly uniform covering 

 (station 5187 ; pi. 28, fig. 4) ; moreover, certain of them transform 

 into spines which are sometimes slender and elongated (pi. 28, fig. 

 6, and pi. 29, fig. 5), but sometimes rather thick and short (pi. 28, 

 fig. 1). In the numerous specimens from station 5520 the greater 

 part of the plates of the dorsal surface of the disk; bear very long r 

 spines which are. rather thick at the base and stongly denticulate 

 throughout their whole length (pi. 29, fig. 1). In figure 8 on plate 

 94 I show the whole series of forms which may be observed between 

 the very short club spines and the very strongly developed true 

 spines like those which are met with in the individuals from sta- 

 tion 5520. 



I have already noted in my original description, published in 1904. 

 the variations which may occur in the armature of the upper and 

 under arm plates. The small conical and short spines which these 

 plates bear may remain slender and little developed, and become 

 localized on their distal border (specimen from station 5275, pi. 27, 

 figs. 6, 7) ; or they may extend over almost the whole extent of the 

 surface of these plates, being sometimes well spaced, and sometimes 

 closely crowded. In figures 2, 3, and 6 on plate 29 I show portions 

 of the arms of individuals from stations 5167, 5187, and 5541, in which 

 these little spines are seen at different stages of development. 



As for the arm spines, these may remain rather short on the dorsal 

 side (specimen from station 5167, pi. 28, fig. 3) ; or they may become 

 more elongate, as is seen in the other specimen from station 5167 

 (fig. 1) and in that from station 5275 (pi. 27, figs. 6, 7) ; or they may 

 become very long, as is the case in the specimens from station 5187 



