OPHIURANS OF THE PHILIPPINE AND ADJACENT WATERS. 285 



arm plates are lacking in the genus Ophiocampsis (a synonym of 

 Ophiogytnna) . 



The diagnosis given by Ljungman of the genus Ophiogymna is 

 therefore correct, seeing that this author does not speak of the pres- 

 ence or of the absence of upper arm plates. But it is necessary to 

 complete this diagnosis by mentioning that the upper arm plates exist 

 in the genus though they are always broken up into numerous frag- 

 ments in the adult, and are only entire in very young specimens, or 

 near the distal extremity of the arms in the adults. His diagnosis is 

 insufficient, and I believe that it is necessary to make it more precise 

 and to complete it by the inclusion of a few other characters. 



The most striking feature of the genus OpMogymna is the great 

 length of the arms and especially the faculty which they possess of 

 rolling up in all planes, often forming circles with a very short 

 radius ; they are aways contorted, and in consequence twisted around 

 each other. Duncan in establishing the genus Ophiocampsis appre- 

 ciated the value of this character and made investigations to find out 

 whether a special structure of the articular facets of the vertebrae 

 did not explain this peculiarity, which is not known in the related 

 genus Ophiothrix. Comparing the articular facets of the vertebrae 

 in Ophiocampsis pellicula and in Ophiothrisc variabilis he recognized 

 the fact that these facets had a more simple structure in the former 

 than in the latter species. " The absence of a peg or its analogue," 

 says Duncan, " the length of the hollow for the umbo and the presence 

 of a pair of nodules characterize this species in reference to the 

 aboral surface of the arm bone. On the other hand, the long umbo 

 and the defective knobs of the adoral surface are just as pecu- 

 liar " (Duncan '86a, p. 118). However, the figures which Duncan 

 published ('86a, pi. 10, figs. 16-20, and pi. 11, figs. 23, 24) are too 

 schematic and somewhat inaccurate. It has seemed to me worth 

 while to take up the question again, especially in view of the notes 

 published in 1913 by Mortensen on the characters of the vertebral 

 articulations in different genera of ophiurans. I have taken as a 

 basis for comparison Ophiothrix pentaphyllum of our coasts of the 

 English Channel, which is very close to Ophiothrix fragilis studied 

 by Mortensen of which that author figured the articular facets ('13a, 

 pi. 2, figs. 8-12). I give photographs of the articular facets of O. 

 pentaphyllum (pi. 103, figs. 6 and &) for comparison with those of 

 Ophiogymna elegans (figs. 7e and /). It may be safely asserted that 

 my photographs of O. pentapJiyllum are quite in agreement with 

 Mortensen's figures, and, considering only the interesting structures, 

 we recognize perfectly on the distal or aboral facet the median 

 tubercle, and on either side the well-known depression or fossa ; on 

 the proximal or adoral facet, the median fossa, and on either side 

 the articular tubercle : these different parts mutually correspond. A 



