126 BULLETIN 84, UNITED STATES NATIONAL MUSEUM. 



of the disk, the folds disappear, the tegument becomes thinner though offering at 

 the same time a few small rounded plates, well separated from one another, and 

 very weakly developed. In the interval which separates the two radial shields of 

 each pair, these plates appear at the height of the middle of the shields; they are 

 at first small and loosely spread, and then they grow larger as the base of the arms 

 is nearer; they even extend to the upper face of the arms on which they very soon 

 disappear. On either side of the radial shields there are also to be found in the 

 interradial spaces some plates which are small and scarce; these plates, besides, 

 appear only near the very edge of the radial shields. These shields are strongly 

 developed. Each of them constitutes an elongated and narrow plate, forming a 

 fairly thick protuberance which is wider distally than proximally, and the length of 

 which equals about half the radius of the disk. The two shields of each pair are 

 far apart and the interval between them is almost equal to the width of the arm; 

 they are directed almost parallel to each other. Every one of these shields is 

 armed with a row of strong, elongated and sharp spines, which are shorter and more 

 dense toward the proximal end of the shield, where they sometimes form two 

 parallel rows. The little plates, which are near the radial shields, are absolutely 

 unarmed. 



The under face of the disk in the interradial spaces is bare; the tegument which 

 covers it is smooth, thin, and transparent. There are, however, near the genital 

 slits, a few small, rounded and scattered plates which become a little more dense, 

 and larger, as they come nearer the mouth shield. The genital slits are very con- 

 spicuous; the genital plates are wide and quite distinct. 



The outlines of the mouth pieces are not very distinct, owing to the tegument 

 which covers them. The mouth shields, which are rather small, are pentagonal, a 

 little longer than wide and a little narrower proximally than distally; they offer an 

 obtuse proximal angle, limited by two straight or slightly excavated sides; the 

 lateral sides are straight and converging, the distal edge is narrow. The elongated 

 and narrow adoral plates are slightly incurved in the shape of a crescent with 

 parallel sides; they are three and a half times longer than wide. The oral plates 

 are well developed, triangular, and very high. As a rule, there are on each side 

 three oral papillae which are strong, conical, blunt-pointed, and have a very rough 

 surface. Besides, there are three or four tooth papillae of the same shape as the 

 foregoing ones, but stronger, and generally arranged in two superposed pairs. 



The arms are of medium breadth and the upper face is convex. The upper 

 brachial plates do not exist. The slight, almost imperceptible calcification of the 

 teguments, which is observed at the basis of the arms and which appears in the 

 shape of small, very thin and rounded plates, which are analogous to those existing 

 between the radial shields, does not go beyond the second article. 



The first under brachial plate is large, trapezoidal, wider than long, with a 

 proximal edge which is wider than the distal edge toward which the lateral sides 

 converge. The following plates are pentagonal, large, much wider than long, with 

 a truncated proximal angle, short lateral sides which are excavated by the ten- 

 tacular pores, and a rounded distal side which sometimes shows in its middle a 

 very small notch. All these plates are contiguous. 



