294 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



single arm. Sometimes all the successive plates are broken up over 

 a rather considerable length of an arm, while sometimes broken 

 plates appear suddenly in the midst of others which are quite entire, 

 and no rule appears to govern these occurrences. Even in the 

 vicinity of the arm tip the upper arm plates may be divided into 

 two or three fragments; in this region they become relatively less 

 broad and their length may then be equal to their width. In general 

 the breaking up of the upper arm plates is not carried very far, and 

 the number of the fragments is not as great as in O. elegam and 0. 

 /pellicula; most commonly the plates are simply divided into several 

 fragments by one or two longitudinal or oblique grooves, as occurs 

 in O. fulgens and 0. pulchella. All the upper arm plates are 

 broadly in contact throughout the whole length of the arm. 



The first under arm plate is rather small, rectangular, and a little 

 broader than long. The following are not very large, and they do 

 not occupy any great part of the ventral surface of the arms. They 

 are at first a little broader than long, with a proximal border a little 

 narrower than the distal, and slightly excavated sides ; at some dis- 

 tance from the disk they become as broad as long. The proximal 

 border is sometimes slightly notched on the first segments, on the 

 following becoming slightly convex; it is the same with the distal 

 border, which is sometimes slightly notched at the base of the arms 

 but which from then on is almost straight. These slightly different 

 forms of the adjacent sides of the under aim plates depend doubtless 

 on the fact that they are sometimes very close together, to the point 

 of being almost in contact, and sometimes separated by a space occu- 

 pied by soft tissue. 



The side arm plates are much developed and projecting, and they 

 carry at least eight arm spines at the base of the arms. The length 

 of these increases from the first ventral to the third, which is slightly 

 shorter than the segment ; the fourth is longer than the segment ; the 

 fifth equals two segments, and the two following are slightly longer 

 still, while the last dorsal spine is shorter than the preceding. These 

 spines are slender and transparent, furnished with asperities which 

 are more or less developed, though not forming true denticulations 

 except in the terminal portion (pi. 103, figs. 9a and e). The fourth 

 to the seventh arm spines have the tip rounded, and the last dorsal 

 spine is pointed. The first ventral spine is transformed into a hook 

 almost at the base of the arms, and the second spine quickly under- 

 goes the same transformation (fig. 9c). 



The tentacle scale is rudimentary or lacking. In certain specimens 

 I have been able to recognize traces of it on the first arm segments, 

 but upon those following it is impossible to distinguish it. 



All the specimens are of an absolutely uniform slightly rosy gray ; 

 the ventral surface of the disk in the interradial spaces is brown 



