1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 289 



The genital slits are long and large, bordered along tlieir distal por- 

 tions by a long conspicuous genital plate. The papilhe are minute and 

 granule-like along the proximal part of the slit, but become larger, 

 flattened, and squarish at the distal end, where they are about two- 

 thirds as long as the upper arm spines. 



The ])apilhe around the fii st tentacle pores are a little larger and 

 blunt; of these there are from six to eight to each pore. The second 

 l)air 01 pores have four or five much smaller papilhe on each side. The 

 third and fourth have about four. The next two have about three on 

 each side; then, on about three or four joints, there are about two or 

 three on the inner side; beyond that, only one. 



The arras are of moderate length, regularly tapered, angular, higher 

 than broad, with a more or less evident dorsal carina. In the typical 

 specimens the dorsal i)lates are very much thickened and prominent; 

 each one is crossed by one, or some'imes two, deep transverse grooves, 

 so that the upper surface is divided into two, and sometimes three, 

 elevations, of which the distal one is the most marked and forms the 

 outer margin of the ])late; the other one forms a more or less irregular 

 central prominence which=at the base of the arms forms a blunt trans- 

 verse ridge, but farther out it becomes a rounded or ovate elevation of 

 the niedian portion only. In the largest specimen the i)rominence at the 

 base of the arms is divided into two by a secondary transverse groove; 

 seen from above the dorsal plates, near the base of the arms, have a 

 more or less regular hexagonal outline. The first seven plates are 

 broader than long; the next six or seven are more regularly hexagonal ; 

 farther out they become more and more elongated, until the length 

 becomes nearly double the breadth. The side arm plates are thick with 

 prominent distal margins. The arm-spmes, which are three (rarely 

 four) toward the base of the arms, are small, short, papilliform, nearly 

 equally spaced, though the upper one is often somewhat removed; they 

 are not more than one fourth as long as the side arm-plates. In one 

 specimen there are regularly four arm spines on about three arm-joints 

 near the edge of the disk. The first under arm-plate is pretty regu- 

 larly pentagonal, about as broad as long; the second and third are 

 larger, longer than broad, with the outer end broadest and the outer 

 margin curved; beyond this the plates become broader than long, with 

 the outer margin strongly curved and the sides slightly convergent; 

 beyond the middle of the arm the form becomes transversely elliptical. 

 Beyond the fifth or sixth under arm-plates the lateral plates meet 

 beneath, and they become relatively longer in proportion as they 

 approach the tips of the arms. 



Variations. — A specimen from station 2528, of somewhat smaller size, 

 having the diameter of the disk 14 mm., differs slightly from the type 

 specimens. The disk scales are more rounded and evenly convex and 

 the characteristic elevations on the dorsal arm-plates are much less 

 conspicuous, owing to the transverse groove being broad and shallow, 

 Proc. N. M. 94 19 



