76 BULLETIN 84, UNITED STATES NATIONAL MUSEUM. 



long. In the large specimens they are contiguous on the first brachial article, then 

 are separated by an interval which is never very long except in the terminal part 

 of the arms. On the specimens the disk of which ranges from 12 to 14 mm. in 

 width, these plates may already be separated from the base of the arms by a narrow 

 interval. 



The first under brachial plate, which is rather small, is triangular with a con- 

 vex distal side on the small specimens, and pentagonal on the larger ones; it has 

 in the latter case two distal sides which meet in an obtuse angle, two straight lateral 

 sides, and a rounded proximal side. The second plate is very large, triangular, 

 almost twice wider than long, or even wider still; in the larger specimens the 

 proximal angle is more or less opened, the distal side is very wide and convex; this 

 second plate is already separated from the first one by the lateral plates. The 

 succeeding plates are pentagonal with a most obtuse proximal angle, straight lateral 

 sides, and a very convex distal side; they very rapidly grow narrower though 

 remaining somewhat wider than long and the interspace between them becomes 

 longer and longer. At a distance of 2 centimeters from the base of the arms this 

 space is equal to the length of the said plates but exceeds it in the second half of 

 the arms. 



The lateral brachial plates are fairly protruding and each of them bears eight 

 spines on the largest samples. These are elongated, rather thin, pointed, and their 

 length increases from the first ventral one, which is longer than the article, to the 

 penultimate dorsal one, which reaches two and a half articles, the last one being 

 often rather smaller than the foregoing one. The surface of these spines may offer 

 rugosities and even conical denticulations, always fairly short, which appear chiefly 

 on the ventral and lateral spines and almost completely disappear on the dorsal 

 ones, the surface of which has simply a rough appearance when seen through the 

 microscope. The two lateral rows of spines always remain separated from each 

 other at the base of the arms, and I do not notice that they are, as Verrill says, 

 "forming an almost continuous band above." The spines become a little shorter 

 and fewer toward the end of the arms, but the length of the dorsal ones remains 

 always almost equal to two articles. 



The tentacular scale is not very large, but it is a little widened at the base of 

 the arms ; it is conical and rather pointed on the smaller examples while it becomes 

 obtuse at the end on the larger ones, and it is often slightly bent; it becomes a 

 little thinner at a certain distance from the disk. Its surface is rough and there 

 are even to be seen at its end a few small spinules on the smallest specimens. 



The color of the specimens in alcohol is yellowish and the upper face of the disk 

 is sometimes a little darker. Verrill says that the color of the live animal is light 

 orange or buff. 



Therefore, to sum up, 0. aculeata shows the following main features : The size 

 is large, since the diameter of the disk reaches 17 mm. in the largest known 

 specimens; the said disk is rounded, not excavated in the interradial spaces, and 

 it is covered by small stumps which are fairly elongated and spinulous. The oral 

 papillae number three on each side, the external one being always widened with 

 the possible intercalation of a supplemental papilla; the number of tooth papillae 

 ranges from one to three. The upper brachial plates are large, triangular, fairly 



