OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 189 



two parts by a transverse groove (fig. 1). The following plates 

 are large, quadrangular, with a straight proximal border, a con- 

 vex distal border, and straight sides; they are at first broader than 

 long, in the terminal portion of the arms becoming a little longer 

 than broad. They are all in contact. 



The side arm plates, which are rather strongly projecting, bear 

 three strong, thick and cylindrical arm spines, with the tips blunted ; 

 their length slightly exceeds that of the segment, and the central 

 one is a little longer than the other two ; the dorsal spine is slightly 

 attenuated. In the distal half of the arms the spines elongate and 

 become relatively a little narrower, while their length reaches a 

 segment and a half. 



The single tentacle scale is very large, oval, and squamiform. 



The color of the specimens in alcohol is gray or sometimes pinkish 

 on the dorsal surface. 



Affinities and distinctive features. — Ophiactis definita is most 

 closely related to 0. perplexa Kcehler found by the Investigator 

 off Cape Comorin in 1,315 meters (719 fathoms), but it is sharply 

 distinguished from it by having the radial shields always sepa- 

 rated, by having the mouth shields longer and much less broad 

 than in O. perplexa, by having the under arm plates markedly 

 broader than long, by having the outermost mouth papilla very 

 much more developed and more pointed, by having the central 

 arm spine more developed, and by having the upper arm plates 

 broad and short. It differs markedly from O. maculosa Martens 

 from the Philippines in having the dorsal plates of the disk less 

 numerous, in having the upper arm plates with rather sharp lateral 

 angles, in having the outermost mouth papilla very much smaller, 

 in having the tentacle scale also smaller, etc. 



Ophiactis definita shows also some affinities with 0. brachgenys, 

 which H. L. Clark described from specimens found in Japan be- 

 tween 120 and 720 fathoms; but the differences are rather impor- 

 tant. The dorsal plates of the disk are more uniform in size, and 

 the radial shields are very much broader in the new species, and 

 the latter are always separated throughout their whole length; the 

 mouth shields are as broad as long, or even a little longer than 

 broad, and the adoral plates are crescentic in form; furthermore, 

 the under arm plates are very regularly in contact in O. definita, 

 and they are never separated by an interval of soft tissue. 



