OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 415 



transverse groove. The adoral plates are large, triangular, broad- 

 ened outwardly, and tapering and pointed inwardly. The oral 

 plates are triangular and much elongated, a shape correlated with 

 the great length of the mouth slits. The mouth papillae number 

 eight or nine on either side, forming a very narrow fringe in which 

 the borders of the individual papillae are only slightly marked, ex- 

 cept those of the two outermost, which are larger and a little broader 

 than the others and rectangular in form. The unpaired terminal 

 papilla is also only slightly developed. 



The first upper arm plate is trapezoidal, broader than long, with 

 the distal border slightly convex; the second, which is in contact 

 with the preceding, is long, trapezoidal, with the proximal border 

 narrower. The following are triangular, and they become rapidly 

 small, though they persist throughout the whole preserved portion 

 of the arms ; they are widely separated from each other and remain 

 a little broader than long. 



The first under arm plate is small, rectangular, almost square, or 

 a little longer than broad. The two following are rather large, 

 polygonal, and widely separated; the second is a little longer than 

 broad, and the third is almost as long as broad or a little broader 

 than long. Beyond this the ventral plates are entirely lacking. 



The side arm plates, which are slightly projecting, bear two very 

 small spines situated close to the ventral border. 



The tentacle pores, which number two pairs, are very small, and 

 are each covered by an extremely reduced scale. 



The color of the specimen in alcohol is a very light j^ellowish 

 white. 



Affinities and distinctive features. — Ophiomusium morio very 

 much resembles O. simplex Lyman, and the arrangement of the 

 plates of the disk is almost the same in the two species with in 

 O. morio a few additional plates separating the radial shields, which 

 in this species are not in contact. It might be supposed that the 

 differences observed depended on the age of the specimens, for 

 O. simplex was described by Lyman from a specimen in which the 

 diameter of the disk was only 6 mm. But H. L. Clark has shown 

 that the dorsal plates of the disk become very numerous in the adult 

 O. simplex, which then assumes an aspect so different that I de- 

 scribed this adult as a different species, to which I gave the name of 

 O. sanctum. If 0. simplex when adult takes on the characters of 

 O. sanctum, it naturally can not retain the arrangement of the dorsal 

 plates that it had when young, and which is exactly what I find in 

 my specimen, the disk of which is 14 mm. in diameter. It may be 

 seen by comparing the photograph which I give here of 0. morio 

 with that which I give of an 0. simplex (pi. 88, figs. 10, 11) of 

 almost the same size (which therefore shows the characters of my 



