396 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



apex; this plate occupies the whole interradial space between the 

 radial shields at the periphery of the disk. Beyond it there is only 

 to be seen the upper border of the large interradial plate, which 

 occupies the whole ventral surface of the disk beyond the mouth 

 shields, with, on either side, one or two very small plates. The radial 

 shields are extremely large, triangular, with a rounded proximal 

 apex, a straight internal border, and a convex external border ; they 

 are twice as long as broad and their length, which exceeds 5 mm., is 

 greater than half the radius of the disk. These shields are slightly 

 divergent and broadly separated for their whole length by the plates 

 which I have mentioned above. Outside of the radial shields are 

 two small tandem plates which on the ventral surface of the disk 

 adjoin the genital plate. 



The ventral surface outside of the mouth shield shows only a 

 single large plate which curves upward at the periphery of the disk 

 to form its lateral surfaces and of which the end is visible when the 

 animal is viewed from above. This plate shows a straight and nar- 

 row proximal border and divergent sides uniting by a very rounded 

 border into the distal side, which is itself rounded; it is a little 

 broader than long. On either side of its distal border is found a 

 small intercalated plate which with its fellow contributes to reduce 

 the length of this border. It sometimes happens also that still an- 

 other smaller plate appears on each side of the distal border of the 

 mouth shield. One of these plates is seen in the specimen from sta- 

 tion 5127, of which a photograph is given in figure 2, but in two 

 interradii only, while in the two specimens from station 5423 these 

 small plates appear almost regularly in all the interradii; in the 

 other specimens they are almost always absent. The genital plate 

 is large, elongated, and rather broad. The genital slits are narrow 

 and sinuous and end at the distal border of the second side arm 

 plate. 



The mouth shields are rather large, longer than broad, pentago- 

 nal, with the proximal angle almost a right angle, the sides at first 

 straight and parallel, then outwardly incurved in such a way that 

 the shield broadens gradually to beyond the middle ; thence the sides 

 become straight and slightly convergent, and they join the straight 

 distal border by slightly obtuse angles. The adoral plates are large, 

 rather long, broadened outwardly, and widely in contact in the me- 

 dian interradial line. The oral plates are triangular and high. The 

 lateral mouth papillae are six in number; the outermost is ex- 

 tremely large and very much longer than broad, but the others are 

 small, rectangular, or square; the unpaired terminal papilla, which 

 is a little stouter than those on either side, is conical and pointed. 



The first upper arm plate is small and very short, trapezoidal, 

 with the distal border rounded. The following remain very small 



