152 Proceedings of the Biological Society of Washington. 



of the paxillse, are absent from the center of the disk and from the 

 median portion of the arms. 



The first superomarginals are slightly wedge-shaped, twice as broad as 

 the abactinal length ; the second are more oblong, twice as broad as long; 

 the following gradually decrease in height so that those beyond the 

 seventh are only very slightly broader than long. In abactinal view the 

 width of the superomarginals remains uniform until the outer third of 

 the arm, whence it slowly decreases to the tip. In one specimen the de- 

 crease in width begins at the base of the arm, while in the smallest it 

 begins at the center of the interbrachial arc. On the arms the supero- 

 marginals are always less than half as broad as the paxillar area between 

 them. 



The superomarginals are evenly convex dorso-ventrally, with the outer 

 surface nearly flat; they are covered with short, truncated, round-tipped 

 spinelets which are not crowded; these become smaller toward the ab- 

 actinal margin, longer and more slender along the lateral borders, and 

 still more slender in the fasciolar grooves. 



The inferomarginals correspond with the superomarginals; they are 

 laterally of the same height and are similar to them. Actinally they 

 form a border of about the same width and relative proportions. Their 

 armature is similar, but they bear on the actinolateral border a stout 

 conical spine which is small and short on the first, slightly longer on the 

 second, and longest on the third or fourth, where it reaches 1.25 mm. in 

 length, thence remaining uniform for some distance and slowly decreas- 

 ing in length toward the end of the arm. 



The actinal intermediate plates are arranged in regular rows between 

 the inferomarginals and the adambulacrals, the rows corresponding to 

 the latter but not to the former. The individual plates imbricate more 

 or less over those preceding. The rows extend as far as the fifth, or 

 proximal part of the sixth, inferomarginal, there being in this distance 

 nine columns, the first of five or six plates, the second of four or five, 

 the third of three or four, the fourth of three, the fifth of two or three, 

 the sixth and seventh usually of two, and the remainder of one. The 

 plates are elevated in the center and bear, on the largest, from fifteen to 

 seventeen short round-tipped spinelets. Each of these groups of spine- 

 lets is separated from those on the plates in the adjacent columns by 

 conspicuous bare channels, and from those of the plates in the same 

 series by similar, but much narrower, channels. 



The adambulacral plates are at first rhombic ; on the arms they become 

 oblong, about twice as long as broad, flattened on the side adjoining 

 the inferomarginals, projecting into the ambulacral furrow in a rounded 

 angle. On the earlier plates the angle in the furrow margin is not cen- 

 tral, but situated near the adoral border; it gradually moves distally, 

 becoming almost central in the plates in the outer half of the arm. The 

 armature consists of six long subequal slender furrow spines; on the 

 earlier plates the second of these from the adoral end lies at the apex of 

 the angle; later the third occupies this position, and on the arms the 

 third and fourth ; beyond these there are two rows of four well spaced 



