174 NATURAL HISTORY BULLETIN 



plates, 12'"°'; radius of disk equals the length of first nine mar- 

 ginal plates; number of superomarginal plates is 48-50, on a 

 side. 



The superomarginal plates proximally are high, angular, short 

 and narrow, about as long as wide above, and encroaching but 

 little on the paxillar area, so that the border of plates is nar- 

 row. Distally these plates become relatively wider, and more 

 rounded, with the sutures oblique. The outer lateral faces of 

 the proximal plates are nearly vertical. The first three or four 

 plates are much compressed and short. 



The inner row of dorsal spines continues only to about the 

 ninth or tenth plate, decreasing rapidly in length to the last, 

 which is a mere tubercle ; the first is a stout, terete, acute spine, 

 about 3™'" high. 



The outer row of spines usually begins on the third or fourth 

 plate and extends nearly to the tip of the rays. These are 

 small, conical, acute, gradually decreasing from the proximal to 

 the distal ones. The upper surface of the plates is covered with 

 elongated spaced granules, or granule-like spinules, largest 

 around the bases of the spines. On the outer surface they be- 

 come much smaller, like fine short spinules, and grade into the 

 longer and more slender f asciolar spinules in the sutural grooves, 

 which are wide and deep. 



The inferomarginal plates scarcely project laterally beyond the 

 upper ones, with subtruncate ends. They have only one large 

 marginal spine, which is rather long, somewhat flatten- 

 ed, tapered or acuminate, and usually very acute. The 

 longer ones are 3"™ long, or equal to the length of three 

 adjacent dorsal plates. Proximally many of the plates have no 

 secondary marginal spine; others have, on the adoral side, a 

 small acute spine not more than a fourth or a third as large. 

 Distally the adoral spine becomes relatively larger and more 



On the ventral side these plates have a transverse row of 

 smaller acute spines along each margin, usually five or six in 

 each, the outer one largest, just at the bases of the marginal 

 spines. Elsewhere the surface is covered with small, semierect, 

 flat, blunt or truncate spinules, not closely appressed, but semi- 

 erect. 



The adambulacral plates have furrow-series of three long, 



