304 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



two branches; these are stout and subequal (pi. 103, fig. 2a). The 

 following spines develop progressively from the second to the fifth ; 

 the elongation is insignificant from the second (<?) to the third (d) 

 and to the fourth (e) ; this last exceeds the segment in length, and 

 the fifth, which is markedly stouter (/), reaches a length of two 

 segments. These spines are smooth over almost two-thirds of their 

 length, and they show in their terminal third numerous very stout . 

 and much elongated pointed teeth which are very close to each 

 other. The general appearance of these teeth with the integument 

 which covers them is that of a well-developed head terminating the 

 spine. The three following spines are smaller and their length as 

 well as their thickness decreases rapidy to the last dorsal, which is 

 short and very slender. These four last spines are united by a thin 

 and transparent wing-like expansion such as occurs in the genus 

 Ophiopteron, which is continued as far as the tip of the arms, though 

 not including more than three spines. This expansion remains 

 strictly limited to the sides of the arms, and does not pass onto 

 their ventral surface. 



The tentacle scale is much elongated and is chiefly formed by a 

 spine rather thick at the base which tapers gradually in such a way 

 as to reach a considerable length (fig. 2b.) 



The general color of the specimen is gray. In the middle of the 

 disk there is a dark blue central spot, which gives off radially five 

 broad bands passing between the radial shields of each pair and- en- 

 croaching upon their internal borders, which continue along the 

 median line of the arms, where their width diminishes. On either 

 side of this dark blue line there is a less well defined line of lighter 

 blue which is slightly marked at the level of the distal border of the 

 dorsal plates. The ventral surface is light gray ; the middle of the 

 arms has the same light coloration, and on either side there runs a 

 rather broad light blue band bounded by the lateral borders of the 

 under arm plates. 



Affinities and distinctive features. — Ophiopteron gratum- is very 

 close to 0. gymnatum, which Macintosh described in 1911 from speci- 

 mens from the Mergui Archipelago. The dorsal plates of the disk 

 have a somewhat different arrangement, but the radial shields are 

 larger in 0. gymnatum and the interradial plates are arranged in 

 several rows. The club spines of the ventral surface of the disk do 

 not reach the dorsal surface at all in O. gymnatum, while in the new 

 species they encroach slightly from that surface. The adoral plates 

 are extremely slender in the species from the Mergui Archipelago, 

 instead of being triangular and rather thick outwardly as in O. 

 gratum; the color of the dark lines which extend jver the dorsal sur- 

 face of the disk and over the arms is quite dif rent ; it is purple in 

 0. gymnatum instead of being blue. These J'nes, though they have 



