212 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



spines which are shorter than those on the rest of the dorsal sur- 

 face; as these club spines are not very closely crowded, the out- 

 lines of the radial shields are very evident. They are triangular 

 and bounded by straight sides, half again as long as broad, and 

 their length equals almost half the radius of the disk. 



The club spines of the ventral surface, which are at first elongated 

 and terminated by several spinules, become progressively shorter 

 and simply conical toward the mouth shields; but they almost 

 entirely cover the whole surface. The genital slits are narrow. 



The mouth shields are slightly pyriform, almost as long as broad, 

 with the proximal angle rather sharp, though with the apex 

 rounded, bordered by two slightly concave sides passing over by 

 very broadly rounded angles into the dorsal side, which is more 

 or less convex. The medium sized adoral plates are triangular, 

 narrowing inwardly, and not in contact in the median interradial 

 line, much broadened outwardly, but not separating the mouth 

 shield from the first side arm plate. The oral plates are extremely 

 broad and not very high. The tooth papillae form two lateral rows 

 between which are found two other median rows which are a little 

 more irregular. 



The upper arm plates are trapezoidal, a little broader than long, 

 with a straight and narrow proximal border, a broad and convex 

 distal border, and divergent sides; the lateral angles are rounded. 

 They are all in contact. 



The first under arm plate is large, quadrangular, almost as broad 

 as long, with the sides convex; this plate is as broad as, and even 

 a little broader than, the following plate. This is longer thaji 

 broad, with the proximal border straight and narrower than the 

 distal border, which is convex; the sides are divergent and con- 

 cave. The third plate is also a little longer than broad, with the 

 distal border convex, and broader than the proximal border. The 

 fourth is almost as long as broad, and, beyond the fifth, the plates 

 become a little broader than long with the proximal border always 

 a little narrower than the distal border, and the latter, instead of 

 being rounded, becomes slightly excavated in the middle. All 

 these plates are broadly in contact. The three first under arm plates 

 are rather deeply excavated, the excavation taking the form of a 

 groove down the middle, while the lateral borders are elevated: 

 sometimes this groove is slightly marked on the fourth plate, but 

 beyond that the surface of the underarm plates remains quite flat. 



The side arm plates, which are slightly projecting, carry at the 

 base of the arms ten arm spines, but this number rapidly falls to 

 eight and then to seven. The three first ventral spines remain very 

 short, shorter than the segment; the fourth is very much longer 

 than the third, and its length much exceeds that of the segment; 



