274 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



size; its arms are incomplete and in a very bad state of preserva- 

 tion ; the diameter of the disk is only 4 mm. The arm spines are 

 almost all torn away, and the dorsal surface of the disk does not 

 have its covering of club spines intact. This specimen is remark- 

 able for the large elongated spot of purplish black which occurs in 

 the middle of each upper arm plate; these successive spots form a 

 very broad longitudinal line which is interrupted at the level of 

 each segment. 



As I stated in 1904 (Kcehler '04a, p. 101, 102), the upper arm 

 plates of the Paris Museum specimens do not have the distal border 

 slightly trilobed as stated by Miiller and Troschel and as Lyman 

 figured it in 1874 (Lyman, '74, pi. 4, fig. 29). I imagine that this 

 trilobed appearance of the distal border is due less to a real in- 

 flection of this border than to two dark red spots which were still 

 to be seen toward the distal border of the upper arm plates when 

 Miiller and Troschel and later Lyman studied the types, but of 

 which there is now no longer any trace, the specimens being com- 

 pletely decolorized. The form of the upper arm plates in the 

 specimens in the Paris Museum, as I find it, does not differ from 

 that shown in 0. stelligera; this can not be invoked, therefore, to 

 separate this species from 0. ciliaris. 



Regarding the club spines of the dorsal surface of the disk, 

 Miiller and Troschel say simply that this surface is covered with 

 granules; Lyman speaks of "minute stumps," and he shows on 

 plate 4 as figure 30 a small narrow club spine ending in a few sub- 

 equal spinules. 



In 1904 I emphasized the form of the under arm plates, of which 

 the distal border is always rounded and passes over to the sides 

 by equally rounded angles; this form, described by Miiller and 

 Troschel, was accurately figured by Lyman (74, pi. 4, fig. 31), 

 and it is easily recognized on the photograph which I give (pi. 

 53, fig. 6). I added on this subject ('04a, p. 102) : " Je crois pouvior 

 poser en principe que toute OpMothrix dont les plaques brachiales 

 ventrales n'auraient pas le bord distal arrondi et convexe n'est 

 point une 0. ciliaris." It is evidently from not having sufficiently 

 studied the form of the under arm plates that certain authors have 

 confused O. ciliaris with other species. 



The under arm plates of 0. stelligera also have the distal border 

 rounded and convex, as Lyman has described and figured it ('74, 

 p. 237, pi. 3, fig. 15), and no more than in the case of the upper 

 arm plates can their shape furnish a character allowing the dif- 

 ferentiation of O. stelligera from O. ciliaris. 



It was for these reasons that in 1904 I was only able definitely 

 to invoke a single character for the separation of these two species ; 

 this was the form of the club spines of the dorsal surface of the 



