the Echinodermata of the Arctic Expedition. 467 



An Ophiacantha with disk covered with small round scales, 

 each bearing a small short spinelet. Radial shields very ob- 

 scure, sometimes quite covered. No disk-incision ; and the 

 dorsal membrane is prolonged over the base of the rays. 

 Mouth-shields twice as broad as long, irregular ovate. Side 

 mouth-shields long, narrow, arched and meeting within. 

 Under arm-plates heptagonal or subheptagonal, breadth equal 

 to length. Dorsal arm-plates triangular. Side arm-plates 

 meeting above and below. Spines 7-8, long, thin, and denti- 

 culate, placed on a keel. 



A greater number of this Ophiuran have been brought home 

 by the expedition than of any other Echinoderm. The speci- 

 mens range in size from those having a disk-diameter of 15 

 millims. to the young form of only 3 millims., and conse- 

 quently form a most instructive series. 



The variations dependent on growth are very considerable, 

 so much so that isolated specimens taken from different stages 

 in the series might easily be regarded as affording the types 

 of distinct species. 



Conclusive proof has been furnished by the material which 

 we have had at our disposal that the 0. grdnlandica, M. & 

 T., and the 0. arctica, M. & T., are untenable species, as Dr. 

 Liitken has already pointed out — and, further, that the charac- 

 ters which had hitherto been regarded as of specific value are 

 not, as that eminent authority seems to infer, even variations 

 such as can be regarded as dependent on distribution, but 

 must be considered simply the phases incidental to age, 

 together with ordinary individual variation. 



Amongst the specimens procured by the naturalists of 

 H.M.SS. ' Alert ' and ' Discovery,' there are many presenting 

 features developed in a manner which might be regarded as 

 " ultraspecific " when compared with the previously recog- 

 nized modifications of this "form." In the present state of 

 knowledge, however, it seems preferable to comprehend them 

 under 0. spinulosa of Miiller and Troschel, rather than to 

 burden further the nomenclature with novel designations. 



The mouth-shields and the under arm-plates in this species 

 are subject to very considerable changes and variation, both 

 in contour and in their relative proportions of length to breadth. 

 In large and adult specimens the number and arrangement of 

 the mouth-papillse is also irregular j and not only is there a 

 frequent increase in number in the ordinal horizontal series, 

 but there is also a great tendency towards reduplication of 

 certain papillas in the vertical axis of the Ophiuran. This 

 seems to arise from the longitudinal cleavage of preexisting 

 papillae. 



32* 



