THREAD-BAYED BRITTLE-STAB. 43 



the breadth of the rays, and arranged in roAvs of about five 

 in each row. The body is dark-reddish brown ; the rays 

 red or flesh-colonr, with a reddish line down the centre of 

 each scale. The ovaries, which are large, and of a bril- 

 liant orange-red, sometimes are seen through the disk, and 

 give it the colouring represented in Midler's figure. The 

 disk of one of my specimens measures three-tenths of an 

 inch across ; and from ray to ray the distance is seven 

 inches. 



The most remarkable peculiarity in this Starfish's or- 

 ganization remains to be described, and has as yet, as far as 

 I am aware, escaped notice either in this or any other species 

 of its tribe. All the spines on each lateral ray-plate resem- 

 ble those of other OpMura, except the third (counting from 

 above). Instead of tapering to a point, as the others do, 

 both in this and in all other described Ojihiocoma, this spine 

 is longer than the rest, and is furnished at its extremity with 

 two transverse-curved spiny processes, giving it exactly the 

 form of a pickaxe. Eight or nine notches are seen on the 

 broad apex. To what end is this singular modification of 

 structure devoted? Why should this species present such 

 a curious armature, and not any of its congeners I The 

 reason is not hidden from us. This OpMocoma lives in 

 soft slimy mud, and thus has to make its way through a 

 very different medium from that inhabited by most other 

 Brittle-stars. The position, the increased size, and the 

 pickaxe heads of these strange spines, are just the modi- 

 fications of structure especially adapted to further loco- 

 motion in such a locality. They may be compared to the 

 lateral hooks or bristles of many earth-boring Annelides, 

 and serve for the same purposes. The contrivance is a 

 very beautiful example of the adaptation of organization 

 to the locality in which the creature is destined to live. 



