318 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



all these specimens from different localities belong to one and the 

 same species. Without speaking of the differences of the kind 

 which Macintosh has so carefully noted ('03, p. 463), the principal 

 variations, always of secondary importance, which I have been able 

 to find in my specimens concern more particularly the form of the 

 mouth shields, and in each of the five individuals of which I include 

 photographs these plates show slightly different outlines; but in all 

 these shields are always broader than long. In the specimen from 

 Roscoff (fig. 5) the mouth shields are quadrangular; the extremely 

 obtuse proximal angle is bounded by two almost straight or slightly 

 convex sides ; the two other sides, which are very convex, are united 

 over an angle so broad and so rounded that together they form a 

 single semicircular border ; the shield carrying the water pore alone 

 has two distinct distal sides which are very slightly excavated and 

 united by an angle which develops into a small median lobe. In 

 the specimen from the Faroe Islands collected by Doctor Charcot 

 (fig. 6) the proximal angle is extremely obtuse and is bounded by 

 very rounded sides ; the two other sides are distinct ; they are slightly 

 excavated, and the distal angle forms a small rounded lobe projecting 

 into the interradial space. On the other hand, in the Sicilian speci- 

 men (fig. 4) the distal border of the mouth shields is semi-circular as 

 in that from Roscoff, while the two proximal sides are united over 

 an angle which projects in the form of a blunted point between the 

 two adoral plates. In a specimen from the Azores (fig. 2) the mouth 

 shields are lozenge-shaped, but less broad and a little longer, rela- 

 tively, than in the others ; the proximal and the distal angle are both 

 very obtuse, though distinct, and almost of the same form. In an- 

 other from the Azores (fig. 3) the mouth shields are broader and 

 the relation between their length and their breadth is almost the 

 same as in those from the Faroe Islands, from Roscoff, and from the 

 Azores ; the form recalls that which I have noted in the Sicilian indi- 

 vidual, with the proximal angle a little less accentuated. The vari- 

 ations which may be observed in the form of the oral and adoral 

 plates are quite insignificant. The arm spines are usually seven 

 in number at the base of the arms ; there are eight on the first arm 

 segments in the Sicilian specimen, which is larger than the others. 



It thus seems to me impossible to establish a specific separation 

 between the examples from the northern region and those of the 

 temperate or warm regions of the Atlantic, and it is evident that 

 0. nigra extends from the coasts of Norway as far as the Azores, 

 entering the Mediterranean where it will certainly be found in other 

 localities than the coasts of Sicily. 



It remains to be seen what should be done with O. tumida. I be- 

 lieve that this name must disappear from zoological nomenclature. 



