166 



OPHIOTHRIX VIOLACEA. 



with a purplish tint ; below, chewing apparatus yellowish, mouth-shields 

 white, speckled with purplish blue ; under arm-plates the same, but, in 

 addition, bordered with pale purplish-blue ; interbrachial spaces finely 

 mottled with white and purplish blue. 



Variations. — The color differs chiefly in being darker or paler ; in 

 the former case, the white spots on the upper arm-plates may be oblit- 

 erated, and even the longitudinal white line become quite faint. Often 

 the under arm-plates are Avhite through the centre, while the lateral 

 sides are dark blue. The proportions of disk to arms present such 

 differences as the following : 9 : 53, 10 : 44, 7.5 : 42, 5 : 23. Dr. Lutken 

 gives a measurement as high as 10 : 80. The average of seven meas- 

 urements of the length of under arm-plates compared to that of longest 

 arm-spines, near base of arm, was .7 : 3.1 ; the extremes were .6 : 3 and 

 .8 : 3.2. The spines on the upper surface of the disk vary somewhat in 

 number and also in the length of their thorns ; those below vary very 

 much in number, being sometimes closely set, sometimes almost want- 

 ing, sometimes thorny, sometimes smooth. The side mouth-shields 

 sometimes quite meet within. 



This species is apparently much more common at St. Thomas and the 

 neighboring islands than in Florida. Certainly in Key West it is com- 

 paratively very rare. It is distinguished from other species as follows : 

 from 0. Orstedii, by short, forked spines of disk, and different pattern 

 of color in alcohol ; from 0. Suensonii, by short, forked spines of disk, 

 and a white median line along arm, instead of a dark one ; from 0. 

 Uneata, by more slender and numerous ann-spines, and by short, forked 

 spines of disk ; from O. angulata, by greater size ( 7°"°' diameter of disk, 

 corresponds in 0. angulata to about 10"""- in 0. violacea), by fewer 

 long spines on disk, and by a much darker pattern of color, when in 

 alcohol. 



LIST OF SPECIAIENS. 



