48 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



little individual did not exceed 3 mm. and the arms were 18 mm. in 

 length. 



The Albatross specimen is much larger, the diameter of the disk 

 being 6 mm. and the length of the arms more than 40 mm. It seems 

 to me best to describe it here as if the species were not yet known, 

 and I believe that it will be preferable from now on to consider this 

 specimen as the type of the species, the Siboga specimen evidently not 

 having acquired its definitive characters. 



The disk is rounded. The dorsal surface is covered with very small 

 thin and transparent imbricated plates which bear extremely small, 

 short and crowded club-spines, of which the length does not greatly 

 exceed the width, and which terminate in three or four very short and 

 slightly diverging spinules. The radial shields are distinct ; they are 

 small, triangular, slightly divergent, a little longer than broad, and 

 widely separated. 



In the interradial spaces the ventral surface of the 'disk shows an 

 investment similar to that of the dorsal surface, but the greater part 

 of the plates are naked, doubtless through abrasion. The genital 

 slits are very broad. 



The mouth shields are of medium size, triangular, almost as long 

 as broad, or a little broader than long, with the proximal angle almost 

 a right angle, slightly rounded, and bounded by two straight sides ; 

 the distal border is slightly convex and shows in the middle a small 

 rounded and narrow lobe; this lobe is already found in the Siboga 

 specimen, which is very much smaller. In the photograph which I 

 reproduce here (pi. 19, fig. 1) the form of the mouth shields is not 

 quite exact because of the rather marked convexity of the ventral sur- 

 face which has led to a slight distortion of the image. The adoral 

 plates are very large, elongated, tapering inwardly, though in contact 

 by a small straight border in the median interradial line ; they are 

 much broadened outwardly and separate the mouth shield from the first 

 side arm plate. It is especially in the elongated form of these plates 

 that the Albatross specimen differs from the immature Siboga indi- 

 vidual. The oral plates are triangular, two and a half times as broad 

 as long. The lateral mouth papillae, three in number, are rather stout, 

 conical and pointed; their size increases slightly from the outermost 

 to the innermost, which is almost as large as the unpaired terminal 

 papilla. Beyond the external papilla, which is inserted like the pre- 

 ceding on the oral plate, there may be distinguished another papilla 

 inserted at the external angle of the first under arm plate which is 

 elongated, slender, pointed, and slightly incurved. 



The upper arm plates are small, triangular, broader than long, 

 with an obtuse proximal angle, two straight sides, and a convex 

 distal border. They are very widely separated from the base of the 



