402 BULLETIN" 100, UNITED STATES NATIONAL MUSEUM. 



The single tentacle scale is oval and larger on the pores of the first 

 pair than on those of the second. 



The large specimen from station 5623, in which the diameter of the 

 disk reaches 27 mm., differs in certain respects from the other speci- 

 mens. The granules of the dorsal surface of the disk are less marked, 

 and the large interradial plate which covers the greater portion of 

 the ventral surface of the disk has the border entirely smooth. The 

 arm spines may reach seven at the base of the arms ; there is always 

 a small ventral group of spines which are subequal and relatively 

 small, and separated from these by a short interval a series of live 

 very short spines, papillif orm and smaller than the two ventral ; this 

 figure falls rather rapidly to four and then to three, and this last 

 number continues over a large part of the length of the arm ; I find 

 it still at 50 mm. from the base. But in the interval I find a few 

 segments which have only two dorsal spines, and consequently the 

 total number of spines is here reduced to four; and even on two or 

 three segments these two dorsal spines are entirely lacking. When 

 viewed from the dorsal surface this large specimen very strongly 

 recalls an O. lymani; it has the dorsal plates of the disk granulose 

 like this last species, and arranged in similar manner. The smaller 

 specimens also show a certain external resemblance to the young 

 of O. lymani, and this resemblance is still further accentuated by 

 the number of the tentacle pores, which are reduced to two pairs ; but 

 the arm spines are more numerous in O. lymani, and the ventral sur- 

 face of the disk in the interradial spaces shows a larger number of 

 plates than in the new species ; in this, as I have stated, the entire 

 ventral surface is occupied by the large interradial plate, which even 

 reaches the dorsal surface of the disk. In O. lymani the plate which 

 follows the mouth shield is very much smaller, and beyond it the 

 ventral surface of the disk shows several other plates. In reference 

 to this point, the photographs which I give of the ventral surface of 

 the disk of three specimens of O. lymani (pi. 86, figs. 7, 8, 9) may be 

 compared with those of this species. 



Affinities and distinctive features. — Ophiomusium facundum is 

 close to 0. validum Lyman, but it can not be confused with that 

 species, from which it differs in having the dorsal plates of the disk 

 rather coarsely granulose, in the somewhat different arrangement 

 of the plates of the ventral surface, in the larger size of the mouth 

 shield, in the arrangement of the arm spines, which are generally 

 four in number, the two lowest close to the ventral border of the sid 3 

 arm plate and almost in contact, forming a small group separated 

 from the two others, while the dorsal spines, which are well separated 

 from each other, are arranged somewhat irregularly and are less 

 constant in their number, and the under arm plates are continued 

 over a rather large part of the length of the arms. 



