286 BULLETIN" 100, UNITED STATES NATIONAL MUSEUM. 



glance at the articular facets of OpMogymna elegans (figs. 7a and b) 

 suffices to make it clear that the tubercles and the articular fossae are 

 almost completely obliterated, and that it is difficult to find any traces 

 of them. Under these conditions we can understand that the articu- 

 lations are very much looser in the latter species, and that the very 

 flexible arms are able to curve about in all planes. 



The vertebral facets thus show a peculiar structure, and it is 

 necessary to mention this character in the diagnosis of the genus 

 OpMogymna. I might even say that this character exceeds in im- 

 portance that which Ljungman invoked for creating the genus 

 OpMogymna — that is, the presence of an integument covering the 

 disk. 



Having found in the Albatross collections two ophiurans discov- 

 ered by the Siboga which I had assigned heretofore to the genus 

 Ophiothrix — 0. fulgens and O. pulchella — and also a new species 

 near these two, which I shall describe under the name of 0. funesta, 

 my attention was attracted again to the forms with flexible arms. 

 Instead of comparing them with other species of OpMothrix, as I 

 had done in 1905, it seemed to me that their affinities were rather 

 toward the genus OpMogymna; moreover, they had quite the appear- 

 ance of the species of that genus. The comparative study of the 

 vertebral articular facets showed me that these three species show 

 characters absolutely identical with those of OpMogymna elegans 

 (see pi. 103, figs. 8g and h and 9/ and g). Considering the external 

 characters, these species have, like the last, very long and contorted 

 arms, and the tentacle scale, -similarly absent in the new species, is 

 very slightly developed and disappears at a short distance from the 

 disk in O. fulgens and 0. pulchella. The tegumentary investment of 

 the disk and of the arms in these three species shows rather wide 

 ■variations. Generally speaking, in O. pulchella this integument is 

 most developed, and the outlines of the plates are least apparent ; in 

 0. fulgens it varies very much individually on the dorsal surface of 

 the disk, but the upper arm plates are always distinct ; it is the same 

 in the new species in which the integument of the dorsal surface of 

 the disk is still less apparent. We find also in these three species 

 a characteristic feature of OpMogymna elegans and O. pellicula, 

 and that is the breaking up of the upper arm plates which always 

 occurs, more or less marked according to the species or the indi- 

 viduals. 



I may add that the arm spines of 0. fulgens and of O. pulchella, 

 like those of the new species, show the same characters as those of 

 OpMogymna elegans; the denticulations, which are very fine over 

 the greater part of their length, develop only in the vicinity of the 

 tip of the spine. A glance at the photographs which I give of the 



