66 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Lateral inferomarginal spines 3, slender, tapering, slightly flat- 

 tened, and sharp, the upper the longest and stoutest, on the proximal 

 plates about 3.5 platts in length (about 5 mm.) ; second spine nearly 

 as long as first ; third about half as long as second, but in very young 

 specimens sometimes relatively longer. The fourth spine is placed 

 as in acanthifer^ but is flattened and longer, being a little over one- 

 half to about two-thirds the length of the longest lateral spine. This 

 spine extends one-half to three- fourths the length of ray. As many 

 as 5 proximal plates may have 2 of these spines, in a cross series. 

 General covering of plate essentially as in acanthifer^ except that 

 there are no accessory spinules in front of the 3 lateral spines as 

 figured by Sladen. 



Adambulacral armature: (1) Furrow spines, 3, relatively long, 

 flattened, slender, round tipped or truncate, the middle the longer, 

 slightly tapering, with edge to furrow; the lateral also slightly 

 tapering with flat side to furrow. (2) Subambulacral spines, usually 

 6, similar in character to furrow spines, slightly tapering and round 

 tipped or truncate, arranged in 2 series, or with 1 in center, sur- 

 rounded b}^ 5; in this case there are 2 spines back of the furrow 

 series, forming an oblique longitudinal series and back of these, 4 

 in a diamond-shaped arrangement. The aboral member of the first 

 series is slightly larger than its companion, and in small specimens 

 ver}^ much larger; in these there are only 2 or 3 subambulacrals, 

 with a minute companion or two. 



Mouth plates very similar to those of A. acanthifer. Any slight 

 differences seem to be covered by individual variation. 



Actinal intermediate plates, 4 in each interradius, with the slender 

 spinelets in two close groups, in some cases forming incipient pedicel- 

 lariae, in others a paxilla. 



Madreporic body small, partly concealed, its own diameter or less 

 from margin; very convex, and with rather coarse transverse striae. 



Color in alcohol; rays with 2 or 3 broad crossbars of brown on 

 bulTy ground: and in addition some specimens have a narrow line 

 proceeding along each radius from near middle of disk to end of ray. 



Young. — Some small specimens, ranging in size from E, 10 mm., 

 to R, 18 mm., have been referred to this form. The raj's are much 

 shorter than in the adult, R equaling 3 to 3.5 r. The paxillae are 

 smaller and less compact, with 1 central and 5 to 8 peripheral spine- 

 lets on the disk and only 4 or 5 altogether on the ray. The supero- 

 marginal plates are well developed as in the adult, the first spine 

 being the most conspicuous. There are 3 lateral inferomarginal 

 spines, but they are stouter and shorter than in the ndult. The fourth 

 spine is wanting except on the first 2 or 3 plates. The subambulacral 

 spines vary in number from 2 to 4 but only 2 are of any size and 1 of 

 these is conspicuously larger than its adoral com|Ninion. 



