372 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



it is somewhat broader than long. The following plates become 

 larger and broader, quadrangular, with the proximal border nar- 

 rower than the distal border, which is always rounded; the lateral 

 borders, which are slightly divergent, are excavated by the tentacle 

 pores. At some distance from the disk these plates become as long 

 as broad, and then longer than broad, while at the same time their 

 proximal border becomes more and more reduced, being finally re- 

 placed by an acute angle. The plates then become lozenge-shaped, a 

 little longer than broad; their length then diminishes and they 

 become separated by a space which elongates little by little; at last 

 they become very small and a little broader than long. The four- 

 first plates have their ventral surface much swollen in the median 

 region, though without forming a true keel as in S. sladeni. 



The side arm plates are very high. They bear spines the arrange- 

 ment of which recalls strongly that in S. stereo. that is to say, the 

 greater part of their free border is occupied by a series of narrow and 

 pointed spines closely crowded against each other and appressed 

 against the lateral surfaces of the arm, of which tlje outlines are 

 very distinct; these are the secondary spines of H. L. Clark. Inde- 

 pendently of these there occurs toward the ventral border of the 

 plate a group of three larger and longer conical and pointed spines 

 which are at first subequal, though the second spine soon becomes 

 broader than the two others. At the dorsal angle of the side arm 

 plates there is another spine, smaller than the preceding, and finally, 

 at some distance from this dorsal spine, toward the upper third of 

 the plate, there is a fifth spine. The secondary spines number about 

 15 at the base of the arms, the number progressively diminishing, 

 while at the same time the primary spines also become reduced, and 

 the number of the ventral spines falls to two, so that in the last 

 third of the arm there remain only four small spines separated by 

 two or three secondary spines. 



The tentacle pores are very large; those of the first pair bear 

 seven or eight scales on either side, and they open immediately below 

 the mouth slits. The following pores are very large over a con- 

 siderable number of arm segments ; their scales, which number seven 

 or eight on the proximal border and six or seven on the distal, de- 

 crease progressively to four; the distal scales then disappear little 

 by little, except for the last, which persists for a considerable time ; 

 the number of the proximal scales remains at six or five over a 

 rather considerable portion of the arms ; the number then falls rapidly 

 to four, to three, and to two. 



Affinities and distinctive features. StegopMura sterllis is close to 

 S. sterea (H. L. Clark) and S. sladeni (Duncan), and it is because 

 of these affinities that I have retained it in the genus Stegophiwra; 

 but it is indubitable that it occurs on the limits of that genus and 



