264 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



tact, even at their distal angle; their internal borders are always 

 parallel with each other for their whole length. 



The ventral surface of the disk is partly naked. Passing over the 

 sides of the disk the dorsal plates become less closely crowded, and 

 when they reach the ventral surface they are entirely separated from 

 each other; these plates, which are unequal, then assume a rounded 

 form and they reach neither the mouth shields nor the genital slits ; 

 their dimensions diminish toward the mouth shields, but this diminu- 

 tion does not occur in a regular manner. Each plate carries a cylin- 

 drical club spine similar to those on the dorsal surface, but the 

 spinules at the tip become shorter and shorter, and finally the club 

 spine ends in a simply rugose point. The genital slits are very broad. 



The mouth shields are triangular, very much broader than long, 

 with a very rounded proximal angle bounded by two straight or 

 slightly concave sides; the lateral angles are rounded and the distal 

 border is slightly convex with a tendency to form a very small lobe 

 in the midde. The adoral plates, which are rather thick, are tri- 

 angular ; they taper inwardly and are not in contact in the median 

 interradial line ; they are much broadened outwardly and send off a 

 process separating the mouth shield from the first side arm plate. 

 The oral plates are very high. The tooth papillae are arranged in 

 five rather regular rows. 



The upper arm plates are large, lozenge-shaped, with the four 

 sides subequal ; they are a little broader than long, with the proximal 

 angle obtuse and the distal angle rounded; the lateral angles are 

 rather sharp. These plates are strongly carinate, and the keel forms 

 a narrow and very prominent line. They are all in contact. 



The first under arm plate is pentagonal, with the proximal angle 

 very rounded and bounded by two slightly excavated sides; the 

 lateral borders are convex and they pass over by very rounded angles 

 to the distal border, which is almost straight. The following plates 

 are large, quadrangular, almost as long as broad, with the proximal 

 border slightly concave, and the distal border convex and rather 

 broadened; the sides are slightly excavated. All these plates are 

 in contact. 



The projecting side arm plates carry ten arm spines at the base of 

 the arms, the number falling to eight and then to seven. The two 

 first ventral spines remain rather short, and especially very slender ; 

 their dimensions then increase rapidly from the third, the length of 

 which equals or exceeds two segments, but which remains rather 

 slender, to the penultimate dorsal, which may reach a length of eight 

 segments; the last spine is shorter. These spines are fine, trans- 

 parent, and provided with rather small pointed teeth which are very 

 close together and regularly arranged. The two lateral rows of 



