156 OPIIIOTHRIX ORSTEDII. 



joint, four small, slender spines, whose length to that of the lower arm- 

 plate is as .8 : .8 ; fourth joint, five spines, 1, 1, 1, 1, .8 : .8 ; fifth joint, 

 five spines, the upper ones much the longest ; sixth joint, nine spines, 

 four upper ones, 2.8, 2.6, 2.2, 1 : .8. Arm-spines of the seventh joint 

 have a length, compared Avith that of the under arm-plate, as follows : 

 (beginning above), 2.8, 2.8, 2.8, 2.8, 2.2, 1.6, 1, .8, .6 :.8 ; there may also 

 be a short supplementary spine on one side above the upper long one. 

 Seventh joint may also have only eight sj^ines ; eighth, eight sphies, 

 four long, four short ; ninth, seven spines, two long, two middling, three 

 short; tenth, eight spines, viz., 2, 2.8, 2.8, 1.6, 1.2, .8, .6, .4 :.8 ; eleventh, 

 seven spines, three long, four short ; fifteenth joint, 2.4, 3.8, 3.8, 1.6, 1.2, 

 .8, .5 : .8. In the neighborhood of the twenty-fifth joint, the spines are 

 only six, and have a length to that of the lower arm-plate as follows : 

 3.6, 3.6, 1.2, 1, .6, .4 : .8. The longer spines, near the base of the arm, 

 have on either edge from four to nine sharj) thorns. The spines on the 

 disk are rather less flattened and more slender, and have one or two 

 thorns on each edge. The spinous rays of the back of the disk in an 

 alcoholic specimen are dark cobalt blue, with a greenish hue ; radial 

 shields the same, but lighter ; tooth-papillae white ; mouth-shields, side 

 mouth-shields, under arm-plates, and side arm-plates, very light cobalt 

 blue, with a greenish tinge, irregularly edged and speckled with white. 

 Upper arm-plates bluish green mottled with white ; between the joints 

 a triple line, consisting of a dark line, with an irregular white one on 

 each side. Arm-spines transparent greenish-blue, with purplish ends. 

 Interbrachial spaces underneath similar to upper surface. 



Variations. — The color in alcohol is about the same as that of the 

 living animal ; it is either bluish or greenish. The young show the 

 oblong scales of the disk very distinctly. 



The white cross lines on the arms easily distinguish this species 

 from 0. magnifica, which, moreover, is larger, and has much stouter 

 spines. It is the most abundant species of Florida and the West 

 Indies. 



