158 OPHIOTHRIX SUENSONII. 



est ; rather thick, with rounded cutting edge. Mouth-shields very wide, 

 short heart-shaped, with outer side straight, and a httle peak within ; 

 length to breadth, .5 : 1. Side mouth-shields short triangular, with cor- 

 ners rounded, meeting within. Under arm-plates as long as broad, 

 outer side longer than inner, corners rounded, a re-entering curve on 

 lateral sides, where tentacle-scale comes ; length to breadth (eighth 

 plate), .7 : .7. Side arm-plates strongly developed, and bearing stout 

 projections to carry the arm-spines. Upper arm-plates about as long 

 as broad ; outer side strongly curved ; inner side straight, and much 

 shorter than outer ; laterals straight and sloping ; length to breadth 

 (seventh plate), .7 : .7. Disk with naked radial shields ; brachial' rays 

 ver}^ narrow ; on them and on the interbrachial rays and centre are 

 scattered spines, long, thin, flattened, usually ending in a rather sharp 

 point, about 2.2"'° in length, with about eight thorns on each edge. 

 Radial shields presenting without an acute angle, terminating in a 

 rounded peak ; length to breadth, 2.5 : 1.4. Interbrachial spaces below 

 with a few spines of same character as those above, but much shorter. 

 Arm-spines five, sometimes six, upper one commonly much the longest ; 

 slender, sharp, somewhat flattened, with about nine thorns on each 

 edge ; ninth joint, five spines ; lengths to that of under arm-plate, 4.5, 

 3.5, 2.8, 1.3, .2 : .7 ; the minute lowest spine becomes a hook very near 

 base of arm. The spines near the end of the arm are extremely long 

 and slender. Tentacle-scales flat, rounded at point, of about same size 

 as lowest arm-spine. Color, in alcohol : above, disk pale purplish, with 

 outer ends of radial shields yellowish ; along outer side of radial shields 

 a narrow, raised edge of white ; a broad band of very dark purple run- 

 ning along each arm, continued along brachial rays, and ending at 

 centre of disk ; side arm-plates mixed yellowish and purple ; below, 

 interbrachial spaces purplish, with fine cross lines of white ; a broad 

 band of light purple, bordered on either side by light, running whole 

 length of arm ; arm-spines white. 



Variations. — In a specimen with a disk of 4°""", the spines on the 

 disk were fewer, and were mostly confined to the centre ; those on the 

 interbrachial spaces below were merely minute points ; the lower arm- 

 plates were longer, and their corners less rounded. In better preserved 

 specimens the ground color was a dull lake-red (yellow ochre and red). 



This species is very distinct from others of America in having the 

 arm-plates as long as broad, and only four or five very long, slender 

 arm-spines. 



