430 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



interradii. The spaces between all these plates are filled with nu- 

 merous extremely small unequal plates which are more or less 

 rounded and not imbricated. The radial shields are rather large, 

 triangular, with the angles rounded, and very much longer than 

 broad; their length exceeds one-third the radius of the disk; the 

 two shields of each pair are somewhat divergent and they are 

 separated for their entire length by several rows of plates. 



The ventral surface is covered with rather large plates which are 

 irregularly polygonal and somewhat unequal. The genital plates 

 are narrow and elongated. The genital slits are narrow. 



The mouth shields are small, narrow, and elongated, pentagonal in 

 shape, at least twice as long as broad, with a sharp proximal angle, 

 and a strongly convex distal border which is continued over very 

 rounded angles into the sides, which on the specimen which I am de- 

 scribing are almost straight ; on other specimens these sides are more 

 or less excavated by the bottom of the genital slits. The adoral plates 

 are rather broad and short, narrower inwardly than outwardly, and 

 they send off a usually very slender process, separating the mouth 

 shield from the first side arm plate. The oral plates are triangular 

 and rather large. The lateral mouth papillae are five in number; 

 the most external is elongated and terminates in- a point which is 

 recurved to follow the outline of the adjacent border of the following 

 papilla ; this is rectangular, very broad, broader than long ; the third, 

 which is smaller, is narrower and almost square ; the two last papillae 

 are conical and pointed. 



The first upper arm plate is short, transversely broadened, from 

 two and a half to three times as broad as long. The second is rectangu- 

 lar, with a straight proximal border, very divergent sides, and a very 

 broad distal border usually broken up into two sides passing into 

 each other by a very obtuse and rounded angle; it is very much 

 broader than long ; the following plates are triangular, a little broader 

 than long, with a proximal angle at first very slightly truncated but 

 rapidly becoming sharp; the distal border ordinarily is broken up 

 into two slightly excavated sides which pass into each other by a very 

 obtuse and rounded angle ; the lateral angles are very sharp. These 

 plates are in contact on the ten or twelve first arm segments, then be- 

 coming separated by a narrow space, and at the same time becoming 

 a little longer than broad. 



The first under arm plate is transversely broadened, very short,, 

 pentagonal, with a very obtuse proximal angle bordered by two 

 straight sides; the lateral borders are short, and they pass over into 

 the narrow distal side by rounded angles. The second plate is rectan- 

 gular with a narrow and straight proximal border, and divergent 

 sides broadly excavated by the tentacle pores ; the distal border is very 

 broad and slightly rounded. On the following plates the proximal 



