68 ON THE ECHINODERMATA OF THE 



each side-plate tapers most. The first fifteen arm-joints bear four spines, the succeeding 

 joints three only. 



An interesting feature connected with this specimen is worthy of record, and is 

 one which does not appear to have been noted by previous observers. The central spines 

 are more or less flattened throughout their whole length ; and at the tip, compression 

 has been carried to such a degree as to form a thin and somewhat expanded head a 

 peculiarity which is at once suggestive of a characteristic spine-appendage possessed by 

 A. Jiliformis ; and although, in the specimen under notice, this structural feature is by 

 no means so fully developed as in that Ophiuran, it is still sufficiently marked to impress 

 upon the mind the near relationship of the two species, and the community of their 

 descent an hypothesis which is also further strengthened by the association of both the 

 forms in more southern waters. 



Description of the Illustrations of this Species on Plate IV. 



Fig. 15. Part of the underpart of the disk : magnified. 



16. Radial shields : magnified. 



17. Side view of the arm : magnified. 



Genus OPHIACANTHA, Mutter & Troschel. 



Ophiacantha, Hiiller & Troschel, Syst. Ast. 1842. 



Disk with little thorny spines. Radial shields covered. Teeth ; mouth-papillae ; 

 no tooth-papillae. Spines numerous, slender, minutely thorny, arranged on the sides of 

 the side arm-plate. Side arm-plates meeting nearly, or quite, above and below. Two 

 genital slits, beginning outside the mouth-shields. (Lyman, op. cit.) 



OPHIACANTHA SPINULOSA, Miiller & Troschel. Plate IV, Figs. 11-13. 



OpMacantha spimdosa, Miiller & Troschel, Syst. Ast. 1842, p. 107. 



An Ophiacantha with disk covered with small round scales, each bearing a small 

 short spinelet. Radial shields very obscure, 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 sub-heptagonal ; breadth equal to length. 

 Dorsal arm-plates triangular. Side arm-plates meeting above and below. Spines 

 7 or 8, long, thin, and denticulate, placed on a keel. 



A greater number of this Ophiuran were brought home by the British Arctic 

 Expedition of 1875-6 than of any other Echinoderm. The specimens range in size 

 from those having a disk-diameter of 15 millims. to the young form of only 3 millims., 

 and consequently 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. grcenlandica, M. & T., and the 0. arctica, 

 M. & T., are untenable species, as Dr. Liitken has already pointed out ; and, further, 



