106 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



similar spines, more closely crowded and more numerous, are found 

 at the periphery of the disk, where they form a rather regular bor- 

 der. In view of the small height of these spines, they might quite as 

 well be given the name of elongated granules. 



The ventral surface of the disk in the interradial spaces runs very 

 obliquely downward because of the strong convexity of that surface. 

 It is covered for the greater part of its area by the large plate which 

 I have noticed above. Following this come two rectangular trans- 

 versely broadened plates, in contact in the interradial median line, 

 and separated from the mouth shield by two or three very much 

 smaller plates placed on each side of the distal lobe of these shields 

 within the ends of the genital slits. These latter are rather large, 

 very evident, and are continued for more than half the height of 

 the large interradial plate. They are bordered on each side by a nar- 

 row plate which throughout its entire length carries extremely fine 

 and very closely crowded teeth which are especially evident under 

 the microscope. 



The mouth shields are rather large, triangular or lozenge-shaped, 

 much broader than long, with a very open proximal angle bordered 

 by two concave sides, and a convex distal border showing a more or 

 less marked rounded lobe in the middle. The adoral plates are 

 large and broad, with the long borders parallel, two and a half times 

 as long as broad. The oral plates are triangular, rather low, broader 

 than high. The lateral mouth papillae are three in number ; the out- 

 ermost is very large, rounded, and squamiform; the two others are 

 slender, elongated, and conical. The unpaired terminal papilla is 

 very broad and thick, and its free border is usually excavated in 

 such a way that the papilla ends in two blunted points. 



The upper arm plates are very large, triangular, very much 

 broader than long, with an obtuse proximal angle and a very con- 

 vex distal side ; their surface is also very convex. These plates are 

 widely separated from the base of the arm outward by an interval 

 which quickly becomes greater than their own length. 



The first under arm plate is large, trapezoidal, a little longer than 

 broad, with the proximal border longer than the distal. The follow- 

 ing plates are very large and occupy the whole width of the ventral 

 surface of the arms ; they are very much broader than long, pentag- 

 onal, with a very obtuse proximal angle bordered by two straight 

 sides, two straight lateral borders, and a slightly convex distal side. 

 They are separated by a space, which, from the third arm segment 

 outward, is greater than their own length. 



The very strongly projecting side arm plates carry at the base 

 of the arms five arm spines ; the dorsal spines on each side come very 

 close together in the median dorsal line on the first segment without, 



