OPIIIOTIIRIX DEMESSA. 



173 



broader than long, oval, with a faint })oint inward ; length to breadth, 

 .7 : 1.1. Side mouth-shields very small and narrow, not meeting within. 

 Under arm-plates small, squarish, with rounded corners, rather broader 

 without than within ; length to breadth, .0 : .G. Upper arm-})lates regu- 

 lar and well marked, much wider than long, wider without than within, 

 lateral corners unusually sharp, outer side cleanly curved ; length to 

 breadth, .6 : 1.2 ; their surface is closely covered with minute thorny 

 spines, like those of the disk, but smaller. Disk closely beset with 

 short, minute spines, which are smooth on the sides, but have a crow n 

 of four to six thorns ; the radial shields also are covered so as to look 

 like the rest of the disk ; below the spines are smaller, more scattered, 

 and more pointed. Arm-spines very slender and elegant, tapering, 

 pointed, uppermost and lowest ones shortest ; on the basal joints com- 

 monly eleven ; lengths to that of under arm-plate, .6, 1, 1.4, 1.0, 1.0, 

 1.6, 1.6, 1.2, .9, .6, .3:. 6. Tentacle-scale small and rounded. Color, in 

 alcohol : above, faint purplish-blue, arms banded with darker, juid a 

 broken stripe of the same running along the middle line ; interbrachiul 

 spaces below same as above ; the rest lighter. 



Variations. — In specimens that have the disk-spines less closely set, 

 the disk is seen to be covered with thin, small, rounded scales ; the 

 outlines of the radial shields, also, may be seen. The upper arm-plates 

 are hexagonal, having the outer side in three lines. The arms in some 

 specimens attain a length twelve times that of the disk diameter. 



Ophiothrix demessa has a very soft, puffed disk, which conmionly 

 wrinkles in alcohol ; it somewhat resembles O. lon(j'q)eda and 0. para^ 

 sita, but has little thorny spines on the upper arm-plates, and is other- 

 wise distinguished. 



LIST OF SPECIMENS. 



