24 BULLETIN" 100, UNITED STATES NATIONAL MUSEUM. 



and divergent. All these plates are separated by a rather narrow 

 interval. 



The side arm plates bear six spines, which form a very regular 

 transverse series; with the exception of the first ventral they are 

 equal, rather short, conical, with the point blunted and armed with 

 strong asperities. The lowest spine is very much more developed 

 than the others, and the difference in size is manifest from the second 

 segment onward, increasing to the fourteenth or fifteenth segment 

 where it is much broadened, thick, although slightly flattened, reach- 

 ing at least a segment and a half in length, while the five other spines 

 do not reach even half the length of a segment. Subsequently the 

 length and the breadth of the ventralmost spine gradually diminish' 

 toward the end of the arm, but it always remains broader and a little 

 longer than the others. The surface of this large spine is covered 

 with rather large asperities at the base of the arm, and these asperi- 

 ties decrease in size as the spine enlarges, reassuming their original 

 development as the size of the spine diminishes. 



On the first arm segment there may be distinguished a very small 

 conical and pointed tentacle scale, which disappears beneath the first 

 ventral spine when this enlarges, but it reappears again when the 

 size of this spine diminishes, that is to say, beyond the fifteenth 

 segment. 



Affinities and distinctive features. — The type of the genus Neoplax 

 is N. ophioides described by Prof. F. Jeffrey Bell from the collections 

 made by the Alert at d'Arros Island, in the Amirante group. In 

 that species the ventralmost spine is a little larger than the others, 

 but it is far from attaining the development which it reaches in the 

 new species in which its size becomes quite diagnostic. 



OPHIOCANOPS, new genus. 



Description. — The disk is small; the arms are very slender and 

 very long, tapering very gradually. The disk is covered with a naked 

 integument without the least indication of plates either upon its 

 dorsal surface or about the periphery. The outlines of the mouth 

 plates are very clear ; the mouth shields are rudimentary, but, on the 

 other hand, the adoral plates are very greatly developed, and are 

 excessively broad. Mouth papillae are present; the teeth are elon- 

 gated and spiniform, arranged in a regular column of six. With the 

 exception of the first, the under arm plates are entirely lacking. The 

 upper arm plates are completely fused with the dorsal surface of 

 the vertebrae, which are divided into two halves by a longitudinal 

 groove, and the two spaces which on the dorsal side of the arm 

 separate the successive vertebrae are occupied by soft parts for a 

 distance almost equivalent to the length of the vertebra. Each of 

 the side arm plates forms a very large prominent projection on the 



