No. IX. ECIIINODERMATA. 279 



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

 Under arm-plates heptagonal or subhept agonal, breadth equal 

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

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

 denticulate, placed on a keel. 



A greater number of this Ophiuran have been brought 

 home by the Expedition than of any other Echinoclerm. The 

 specimens range in size from those having a disk-diameter of 

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

 quently furnish 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. gronlandica, M. and 

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

 Dr. Liitken has already pointed out — and, further, that the 

 characters 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 distri- 

 bution, 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- 

 nised modifications of this 6 form.' In the present state of 

 knowledge, however, it seems preferable to comprehend them 

 under 0. spinulosa of Muller 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-papillae is also irregular ; and not 

 only is there a frequent increase in number in the ordinal 

 horizontal series, but there is also a great tendency towards 



