106 OPHIACTIS SIMPLEX. 



outer side of mouth-shield to inner points of teeth, to that between 

 outer corners of mouth-sUts, 1 : .9. Mouth-papillae, two to each angle 

 of mouth, situated near outer end of mouth-slit, about as broad as long, 

 thin, angular. Cutting edge of teeth re-enteringly curved. Mouth- 

 shields broader than long, nearly oval, with a slight peak within ; length 

 to breadth, .4 : .5 ; madreporic shield much larger. Side mouth-shields 

 very large, about as broad as the length of the mouth-shields, re-enter- 

 ingly curved at the outer corners of the mouth-slits ; meeting fully 

 within, and also touching their neighbors without, so as to cover the 

 first under arm-plate. Under arm-plates even, a little overlapping, 

 about as broad as long, rather thick ; outer side cleanly curved, laterals 

 short and nearly straight ; within there are an inner side and two inner 

 laterals, which are short and nearly equal, and are often more or less 

 combined in a broken curA^e ; length to breadth (tenth plate), .5 : .4. 

 Side arm-plates small, making a narrow, prominent ridge for the arm- 

 spines. Upper arm-plates regular, a little overlapping, much broader 

 than long ; bounded without and within by straight lines, and on the 

 sides by curves ; a little narrower within than without ; first two or 

 three plates much shorter than those beyond ; length to breadth (sev- 

 enth plate), .5 : 1. Disk-scales pretty regular and rounded, and rather 

 thick ; in the centre a conspicuous primary plate, with an indistinct 

 rosette about it ; in each interbrachial space a radiating, connected row 

 of rounded primary plates, between which and the radial shields there 

 are two or three pretty regular rows of smaller scales, about 16 to a 

 square mm. Radial shields small, pointed within, widely separated by 

 a wedge of difierent sized scales ; length to breadth, .8 : .4. Along edge 

 of disk, and on interbrachial spaces below, numerous short spines, of 

 Avhich there are also a few scattered about the back of the disk. Arm- 

 spines five, only moderately stout ; round, tapering, blunt, slightly 

 rough ; three middle ones longest, lowest one shortest ; lengths to that 

 of under arm-plate (sixth joint), .3, .4, .4, .4, .2 : .4. Tentacle-scale much 

 longer than broad, nearly oval, more than half as long as the under 

 arm-plate. Color, dry : alcove, and interbrachial spaces below, very 

 dark olive-green ; rest of lower surface yellowish. 



Variations. — Of fourteen specimens that I have examined, the two 

 largest had only five arms ; the rest, which were much smaller, had six ; 

 except one small specimen, which had but four. A 3'oung one had the 

 following characters : diameter of disk, 1.2'"'"- ; length of arm, 4.2"'"-. 

 Upper arm-plates, most of them, separated by side arm-plates, heart- 

 shaped, somewhat swollen ; disk covered with large, rounded, swollen 

 scales, of which the eight middle ones form a rosette ; radial shields 

 not conspicuous, and smaller than most of the disk-scales ; a very few 

 minute spines on the disk ; arm-spines four, more slender than in the 



