110 OPHIACTIS MULLERL 



short outer laterals, and on the sides by straight laterals, all the corners 

 being somewhat rounded; length to breadth (eighth plate), A : A. Side 

 arm-plates stout, regular, encroaching a little below. Upper arm-plates 

 cleanly and regularly oval, stout, but not SAVollen, slightly overlapping ; 

 length to breadth (third plate), .4 : .7. Disk-scales, near the margin, 

 about 50 to a square mm. ; considerabl}^ smaller below, but larger near 

 the centre, above ; a very uneven rosette of primary plates in the mid- 

 dle of the disk, and usually one elongated plate in each brachial and 

 interbrachial space ; near edge of disk, a few very short spines. Eadial 

 shields either touching for their whole length, except their inner points, 

 or else more or less separated by one or two very narrow scales ; length 

 to breadth, .9 : .4. Arm-spines four, second one from the top much the 

 stoutest and roughest, lowest one smallest, the other two intermediate 

 in size ; all are nearly cylindrical, and somewhat tapering ; lengths to 

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

 one, very similar in shape and size to the mouth-papilla. Color, in 

 alcohol : upper surface and interbrachial spaces below variegated Avitli 

 sap-green, dull cobalt blue and white, the principal tint being green ; 

 remainder of under surface nearly white. The color is about the same 

 in the living animal. 



Variations. — The above-described specimen had only five arms, but, 

 as a rule, the number is six, and of these two or three are almost always 

 smaller than the rest. The arms vary much in numlDer ; occasionally a 

 specimen may be seen with only one. A young one had five arms 

 about 3.5'"'°- long, the diameter of the disk being 1.2™""-; lower arm- 

 plates long pentagonal, wdth an angle inward ; upper arm-plates broad 

 heart-shape, with a curved outer side, and straight inner laterals ; they 

 had a peak within, and were completely separated by the encroachment 

 of the side arm-plates ; upper side of disk without any spines, and cov- 

 ered by the radial shields, and a few large, irregular scales, some of 

 which formed a sort of rosette in the centre. It is said that there are 

 sometimes five arm-spines, but I have never seen more than four ; and 

 many of the basal joints have but three. 



O. MUlleri may be recognized by its single mouth-papilla, oval upper 

 arm-plates, and six arms. 



It has been found abundantly at St. Thomas (A. H. Riise). At Cape 

 Florida I found great numbers, living in colonies, at low-water-mark. 

 The little animals were clinging to a kind of spreading, greenish sponge, 

 which grows like a thick net-work, and often attains a thickness of a 

 foot, and a diameter of four or five feet. 



