318 VERRILL 



into the small spinules of the fascicles on the sides. At the summit 

 of more or less of the proximal plates there is a small, nipple-shaped 

 or thimble-shaped spine, which is liable to be broken off, leaving a 

 small, pit-like scar. 



The dorsal paxillse bear small, regular, stellate groups of short, 

 somewhat unequal, clavate or capitate spinules. In the larger ones 

 there is usually a central, larger, capitate spinule, surrounded by a 

 circle of about six to eight smaller ones ; these are surrounded by a 

 marginal circle of about ten to fifteen of about the same size. 



The lower marginal plates are strongly spinose ; most of the sur- 

 face is covered with small, flat, more or less imbricated spinules. 

 There is a submedian row of six to eight unequal, roundish, acute 

 spines, increasing in size outwardly, the last, or next to the last, being 

 much the largest, stout and acute, not much flattened; sometimes 

 there is a smaller slender one above the largest. 



Adambulacral plates have a convex inner or furrow-group of 

 about four slender spines, of which the innermost is larger, longer, 

 angular, and stands in front of the others. On the actinal side there 

 is a central, erect, unusually large, flat, blunt or truncate, often 

 grooved spine, and back of this a row of three or four much smaller 

 spines, of similar shape, on the outer and proximal margins of the 

 plate. 



The adoral and epioral spines are very numerous, short, stout, 

 obtuse. 



It varies greatly in the length and sturdiness of the rays, in the size 

 of the superomarginal plates, and especially in the number and form 

 of the tubercles or spines on the superomarginal plates. These may 

 be reduced to a single imperfect series of small tubercles, or there 

 may be one continuous series, or two imperfect series rarely two 

 nearly complete series. They may be lacking proximally and pres- 

 ent distally, or vice versa. Sometimes they become spine-like, espe- 

 cially the outer ones ; but more often they are short, stubby cones or 

 blunt tubercles. Other parts also vary. Generally, however, the 

 enlargement of the central spinules of some of the paxillae, the 

 differentiation of the paxillae along the median line, and especially 

 the scale-like, flattened, inferomarginal spinelets, remain pretty 

 constant. 



This species, in form and general appearance, resembles A. cer- 

 stedii Liitken, of Central America, which is now generally considered 

 identical with the true armatus of Gray. This resemblance is due 

 in large part to the character of the marginal spines in the interradial 



