356 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



middle of which is continued into a small prolongation ending in 

 an obtuse angle which reaches the dorsal surface of the disk between 

 the radial plates. The lateral borders of these shields are divided 

 into two sides one, which is proximal and straight, is very short: 

 the other, which is elongated, is slightly concave. The genital slits 

 are scarcely distinguishable, and they extend along about a third 

 of the length of the corresponding shield. The adoral plates are 

 much developed, thick, with the parallel borders slightly incurved; 

 they are rather short, but very broad; their surface is slightly con- 

 vex. The oral plates are small and triangular. The mouth papillae 

 are not very distinct ; I can make out, however, on either side three 

 low rectangular papillae which are very much broader than high, 

 and one unpaired terminal papilla, which is rather short and 

 broadened. 



The first upper arm plate is extremely small and very short, in 

 the form of a very depressed arc of a circle. The following are 

 larger, though always remaining of small size and widely sepa- 

 rated; they are triangular, broader than long, with a very obtuse 

 proximal angle, straight sides, and a very slightly convex distal bor- 

 der. They extend as far as the tip of the arms, though becoming 

 extremely small terminally. 



The under arm plates are more developed than the upper; their 

 size diminishes progressively from the first outward. This is rather 

 large, pentagonal, a little longer than broad, with an obtuse and 

 slightly rounded proximal angle, sides strongly excavated by the 

 corresponding tentacle pores, and a slightly convex distal border. 

 The four following plates have a similar form, but their dimensions 

 progressively decrease and their sides are less strongly notched by 

 the tentacle pores ; they are widely separated. Beyond the fifth seg- 

 ment the tentacle pores disappear and the plates are quite rudimen- 

 tary; from this point they disappear rapidly. 



The side arm plates, which are very large and elongated, are 

 broadened and projecting in their distal portion, which carries two 

 extremely small and rather separated arm spines situated toward 

 the middle of their height. 



There are four pairs of tentacle pores which are rather large, 

 especially those of the two or three first pairs. On the pores of the 

 first pair there may be distinguished one or two scales on each 

 border; the pores of the second pair usually still have one scale on 

 either side, but the following are without them. 



The color of the specimen in alcohol is a rather dark brownish 

 yellow. 



Affinities and distinctive features. I believe it necessary to refer 1 

 this species to the genus Ophiopyrgus rather than to the genus 



