224 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



the outermost only being well developed. I also notice that the 

 ventral surface of the disk is in part covered with club spines identi- 

 cal with those of the dorsal surface in 0. crassispina, while in O. 

 cu77iulata it is almost entirely naked ; but this character has no great 

 value. 



I may add that the coloration of these two species may be quite 

 different in the living animal, at least judging from the specimens in 

 alcohol ; these are quite white in O. crassispina, while they all retain 

 a grayish-yellow coloration in O. cumulata. 



For these reasons I believe it necessary to separate O. cumulata 

 from 0. crassispina and to consider that the remarkable transforma- 

 tion that the dorsal spines in certain specimens of the former undergo 

 is fortuitous and does not constitute a specific character. 



We must therefore inquire to what cause may be due the thicken- 

 ing of these spines. The first idea which naturally comes to mind 

 is that this thickening is induced by a parasite, especially when one 

 recalls the modification described by Mortensen and by myself which 

 the spines of certain Antarctic species of Clenocidaris and Rkyncho- 

 cidaris undergo under the influence of a parasite called by Morten- 

 sen Ecliinopliyces vmrabilis (see Mortensen '10, p. 12; Mortensen 

 and Kolderup Kosenvinge '10, p. 339; Koehler '12, p. 154). But this 

 is not the case. I have examined under the microscope a number of 

 sections made of the thickened spines of 0. cumulata and I have 

 not been able to observe the least trace of a parasite. These spines 

 are solid; they are formed of a perfectly regular calcareous sub- 

 stance composed of rods radiataing from the center to the periphery, 

 and they do not show the least trace of foreign bodies, as may be seen 

 in the photograph which I give here (pi. 50, fig. 4). 



The thickening of the dorsal spines in certain specimens of 0. 

 cumulata seems to be rather to be compared with that known in cer- 

 tain species of OpMocoma, and in particular in O. scolopendrina. 

 In this species some specimens have the last dorsal spine very short, 

 thickened, and strongly swollen, while in others this spine is scarcely 

 modified in its form, or even remains quite cylindrical and elongated. 

 This modification of the dorsal spine in 0. scolopendrina and in other 

 related species 9 is quite individual and has no specific significance 

 The only difference to be remarked is that the thickening of the 

 dorsal spine shows itself somewhat irregularly in the specimens of 

 OpMocoma in which it occurs, while in 0. cumulata when the modi- 

 fication is produced it involves all the successive dorsal spines of the 

 bases of the arms, which all show, over a considerable distance and 

 in the most regular manner, the same transformation. 



8 I shall notice further on the similar variations in the two dorsal spines of OpMocoma 

 lineolata. 



