170 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



The dorsal surface is uniformly covered with small, imbricated, 

 subequal, rounded plates, becoming a little larger in the vicinity of 

 the radial shields and a little smaller towards the periphery of the 

 disk; there is not the slightest indication of primary plates. The 

 medium-sized radial shields are triangular, with a straight internal 

 border and a convex external border; they are two and a half times 

 as long as broad. The two shields of each pair are only in contact 

 by their external angle, diverging rather strongly inwardly; the 

 length of these shields is less than half the radius of the disk. 



Passing to the ventral surface of the disk the plates become very 

 much smaller and especially extremely narrow ; they are imbricated, 

 very closely crowded, and retain the same form over the whole extent 

 of the ventral surface. The genital slits are very broad. 



The medium-sized mouth shields are large, longer than broad, 

 pentagonal, with the proximal angle very broadly rounded, bounded 

 by two somewhat sinuous and slightly divergent sides which pass 

 over strongly obtuse and very rounded angles to the two lateral 

 borders ; these are a little shorter and gently convex ; the distal bor- 

 der is narrow, almost straight or slightly rounded. The adoral 

 plates, which are of medium size and short, are triangular, broader 

 outwardly than inwardly, where, however, they are still rather 

 broad, and terminate in a rounded angle; they are not quite in con- 

 tact in the median interradial line. The oral plates are small and 

 rather low. The lateral mouth papillae, four in number, are not 

 greatly developed ; the proximal papilla is rather stout, conical, with 

 the tip rounded; the following is small and conical; the third is 

 larger and broader, and rounded; the fourth is very much smaller 

 and still more rounded. These two last are inserted on the adoral 

 plate. 



Compared with the diameter of the disk, the arms are narrow and 

 slender. The upper arm plates are not very broad, but the under 

 arm plates are extremely narrow. The upper arm plates are half 

 again as broad as long, triangular in shape, with the proximal angle 

 broadly rounded, and a very convex distal border; their form may 

 even become biconvex ; they are all in contact. 



The first under arm plate is small, quadrangular, broader than 

 long. The following plates are, as I have just said, especially nar- 

 row; they are pentagonal, longer than broad, with a very obtuse 

 and broadly truncated proximal angle, straight sides, and a slightly 

 rounded distal border ; they are all in contact. 



The well-developed side arm plates carry at the base of the arms 

 six spines which are almost equal in size ; the two first, however, are 

 the largest, then the size decreases to the last dorsal which is shorter 

 than the arm segment. These spines are slightly flattened, and much 

 broadened in their proximal half ; they taper rather rapidly in their 



