258 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



club spine of which the tip is rounded and slightly broadened, and 

 which is provided with a few very short and very fine spinules. 

 These club spines become a little shorter toward the periphery of 

 the disk, while on the other hand their spinules become a little more 

 developed. In the radial spaces, which are always very narrow, the 

 club spines are very much rarer and when they occur they are ex- 

 tremely short. The radial shields are very large, triangular, half 

 again as long as broad, and their length is almost equal to three- 

 quarters of the radius of the disk. The two shields of each pair are 

 very close together, though not in contact, and they scarcely diverge 

 at all, the two straight inner sides being almost parallel ; the external 

 border is somewhat sinuous, and the proximal angle is rather sharp. 

 The surface of these shields is entirely naked. 



The ventral surface of the disk is occupied in part by very small 

 rounded plates each of which bears a club spine identical with those 

 on the dorsal surface, the base of which occupies almost the entire 

 surface of the plate which carries it. These plates cover a triangular 

 space which reaches neither the mouth shields nor the genital slits. 

 These latter are rather broad. 



The mouth shields are large, triangular, broader than long, with 

 a rather prominent but rounded proximal angle bounded by two 

 slightly concave sides; these pass over by rounded angles to the 

 distal border, which is convex and shows in the middle a very small 

 rounded lobe. The adoral plates, which are of medium size, are tri- 

 angular and their proximal apex is very close to the median inter- 

 radial line, but it does not quite reach its fellow; the very broad- 

 ened base of these plates sends off a narrow process which separates 

 the mouth shields from the first side arm plate. The oral plates are 

 rather broad, but only slightly elevated. The tooth papillae form two 

 very regular lateral rows, bounding three other rows which are very 

 much more irregular. 



The rather small upper arm plates are as broad as long, or even 

 slightly longer than broad. They are almost lozenge-shaped, with 

 a truncated proximal angle bounded by two straight sides ; the two 

 other sides are slightly convex and they are united over a rather 

 obtuse and rounded distal angle. All these plates are in contact. 



The first under arm plate is rather small, rectangular, with the 

 sides convex ; the proximal border, which is slightly rounded, is nar- 

 rower than the distal border, which is straight. The following 

 plates are large, rectangular, longer than broad, with the proximal 

 border straight and a little shorter than the distal border, which is 

 slightly concave ; the sides are somewhat divergent and slightly exca- 

 vated ; the lateral angles are rounded. All these plates are in contact. 



