120 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



in each of the interradial spaces one of them is noticeable on account 

 of its very much greater dimensions. A certain number of plates, 

 and especially the large central plates, carry each a very large 

 cylindrical and thick spine which remains of the same diameter to 

 its rounded extremity; the surface of these spines is very rugose, 

 and their length equals or even exceeds the diameter of the plate 

 which carries them. My specimen has retained nine of these large 

 spines, and I recognize the scars of a few others which must have 

 been torn off. Other smaller spines occur here and there on the plates, 

 and they finally pass into large elongated granules, with the ex- 

 tremity slightly swollen, which are seen on the periphery of the disk ; 

 these granules, like the large spines, are covered with very closely 

 crowded asperities. All these dorsal plates show a narrow and very 

 finely denticulated marginal border, less apparent and less broad, 

 however, than in the preceding species. The radial shields are of 

 almost the same dimensions as the large central plates ; they are tri- 

 angular, with the angles and the sides rounded; they are almost as 

 long as broad, and their length is almost equal to one-fourth the 

 radius of the disk; the two shields of each pair are in contact 

 throughout their whole length. 



The ventral surface of the disk is covered with rather large plates 

 similar to those on the dorsal surface, but a little smaller, and show- 

 ing, like them, a narrow and denticulated border; their dimensions 

 diminish toward the mouth shields. The very broad genital slits 

 extend from the mouth shield to the periphery of the disk. 



The mouth shields are rather small, triangular, very much broader 

 than long, with a rather prominent proximal angle bordered by two 

 straight or slightly concave sides, and a rounded distal border; on 

 two of the shields this border shows in the middle a small rounded 

 lobe. The adoral plates, which are much elongated, have the long 

 sides almost parallel ; they are, however, a little broader distally, and 

 often give off a narrow process which separates the mouth shield 

 from the first side arm plate. The oral plates are triangular, and 

 almost as high as broad. The lateral mouth papillae, three in num- 

 ber, are small, cylindrical, half again as long as broad, with the tip 

 rounded; their surface is covered with very strong asperities. The 

 unpaired terminal papilla is much broadened and obtuse. 



The first arm segments are rather short ; the following very rapidly 

 elongate, and the arms then become slightly moniliform. The upper 

 arm plates, which are of medium size, are triangular, with an obtuse 

 proximal angle bordered by two straight sides, and a very convex 

 distal border ; they are a little broader than long and separated from 

 the base of the arms outward by a progressively increasing interval. 



