36 BTJLLETIN 84, UNITED STATES NATIONAL MUSEUM, 



are at first wider than long, they afterwards become as wide as long. They extend 

 up to the ends of the arms and are always very widely separated through the lateral 

 plates. The granules which are to be seen on their surfaces are very fine and much 

 smaller than on the lateral plates; there is no indication of tubercles such as those 

 which are known to exist on 0. pulcheUum. 



The first under brachial plate is large and sensibly longer than wide. It is nar- 

 rowed in its proximal half by the corresponding tentacular pores, while it widens 

 distally; ithas an obtuse proximal angle which is more or less rounded, lateral borders 

 which are veiy concave and a distal side which is widened and convex. The following 

 plates grow rapidly smaller, and they are widely separated by the lateral plates. 

 The form of the second plate resembles that of the first, but it is narrowed to a 

 greater extent in its proximal half, whUe it widens more in its distal region. This 

 form becomes exaggerated on the succeeding plates, the proximal parts of which 

 finally disappear altogether, when the plates assume the shape of a triangle with an 

 obtuse proximal angle and a most convex distal side. These plates, although much 

 reduced, extend to the ends of the arms. 



The lateral plates acquire a very great development chiefly on the imder side 

 where the first two, principally, are considerably widened. The first plate is short 

 and very wide; the second, almost as wide as the preceding one, is somewhat longer; 

 the tliird and the fourth, still narrower, grow longer, and the length rapidly increases 

 in the following plates, while the width decreases more and more, so that the plates 

 are finally much longer than wide. The first lateral plate carries two faMy strong 

 spines, short and flattened, truncated at their ends and with some roughness on 

 their borders; on the second plate the spines, amounting to three, have the same 

 shape as the preceding ones, but they are longer and stronger; the third plate bears 

 two or three spines, and the succeeding plates have only two spines, which rapidly 

 become very small. 



The surface of the first lateral brachial plates is strongly granulous. The 

 granules, larger than on the other plates, are rounded and widely separated; they 

 grow finer as they He nearer the arm ends. 



The tentacular pores amount to five pairs on each arm, and they are provided 

 each with one fairly large, rounded scale. 



Connections and differences. — 0. speciosum is closely allied to 0. pulcheUum 

 (Lyman), from wliich it differs, however, first in having the upper plates of the disk 

 and arms completely deprived of these tuberosities which impart to the upper face 

 of 0. pulcheUum such a characteristic appearance, and also in having the inter- 

 radial imderspaces more widened, owing to a lesser development, in breadth, of the 

 first brachial articles. The mouth shield is succeeded by a single median plate; 

 this plate, much elongated and pretty wide, extends from the mouth shield to the 

 margin of the disk and separates the two genital plates on their whole length, 

 which is not the case in 0. pxdchellum, which may offer either the arrangement 

 indicated by Lyman or the one described as 0. pvichellum from station 2625. 



