360 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



there is only a single row of very much narrower interradial plates. 

 The central rosette is irregular, for the dorso-central plate is replaced 

 by four other very small plates, and one of the primary radial plates 

 is itself divided into two. These radial plates are rounded, a little 

 broadened transversely, and they measure about 2 mm. in width. Out- 

 side of the circle which they form there is another circle of larger 

 interradial plates, which are oval and radially elongated, and separate 

 the proximal angle of each radial shield from the corresponding angle 

 of the shield of the neighboring pair. Each of these plates is fol- 

 lowed by another a little larger, usually triangular in shape, with the 

 base very close to the periphery of the disk. Following this last there 

 is a large plate which delimits the vertical borders of the disk between 

 the genital plates in each interradial space ; a very small part of this 

 plate is seen when the animal is viewed from above. Between this last 

 and the triangular plate which precedes it there is on either side a very 

 small intercalated plate ; between the first and the second interradial 

 plates also there is a group of two very small plates. In the radial 

 spaces, between the primary radial plates and the radial shields, there 

 is only a rather constant group of three very small plates of which the 

 median, triangular in shape, separates the two radial shields of each 

 pair at their proximal extremity. All these plates are projecting and 

 strongly convex, and they are separated from each other by rather 

 deep grooves ; their surface is perfectly smooth and very shiny. The 

 radial shields are very large and form a broad peripheral ring which 

 covers a large part of the dorsal surface of the disk. These shields are 

 elongated, triangular, with the borders and angles very rounded ; they 

 are half again as long as broad and are in contact throughout almost 

 their entire length, which, reaching on the average 4.5 mm., exceeds 

 half the radius of the disk. Their surface is very convex and per- 

 fectly smooth like that of the other plates of the dorsal surface. The 

 radial papillae are cylindrical, rather short, with the point obtuse, and 

 very close to each other. 



The ventral surface of the disk in the interradial spaces includes 

 only a small number of plates. Usually two paired plates, which are 

 rather small and rounded, suffice to cover the space between the mouth 

 shield, the genital plates, and the large interradial plate which limits 

 the sides of the disk and encroaches somewhat on the ventral surface. 

 Sometimes one or two extremely reduced plates are found at each 

 external angle of the mouth shield. The genital plates are of medium 

 size. The narrow and elongated genital slits are provided along their 

 whole interradial border with a range of low and obtuse papillae. 



The mouth shields are large, elongated, and their distal half occu- 

 pies the whole width of the ventral surface of the disk between the 

 genital plates. They are longer than broad and pentagonal in shape, 

 with a rather sharp proximal angle bordered by two small straight 



