158 THE VOYAGE OF H.M.S. CHALLENGER. 



are confined to the adoral or inner two-thirds of the plate. Three to five small aborted 

 spinelets, little more than granules, stand upon the surface of the plate behind and 

 external to the furrow series, and form an aborted secondary series. The adambulacral 

 plates are separated throughout the ray from the marginal series by a narrow strip 

 of membrane with scale-like plates continued from the actinal interradial area. 



The mouth-plates are large, prominent along the line of suture, forming a broad well- 

 elevated keel in which nearly the whole of the two plates is involved. The aboral extre- 

 mity slopes gradually, the surface of the actinal interradial area being inclined upward to 

 meet it, and adorally they also likewise slope gradually. The sutural junction is imper- 

 fect, and widely expanded aborally, exposing the odontophore. The armature of the 

 mouth-plates consists of six mouth-spines proper on each plate, i.e., twelve for the whole 

 mouth-angle ; the innermost one is much larger and more robust than the others, and 

 there are thus two large spinelets at the innermost point of each mouth-angle directed 

 over the actinostome. The five smaller spinelets, which are uniform, equal, and less than 

 the spines forming the armature of the adambulacral plates, are arranged equidistantly 

 along the lateral margin of the plate, and arch over the furrow in continuation of the 

 spinelets on the adambulacral plates. About three aborted secondary spines are placed 

 close to the margin of the plate, which falls in the median suture ; the middle one is 

 largest, subconical, and stands nearest the highest point of the keel ; the most adoral 

 is similar in shape, but rather smaller, and is placed midway between this and the 

 anterior extremity ; whilst the outermost one is little more than a tubercular granule, 

 and stands equidistant between the middle spine and the aboral extremity. Occasionally 

 the adoral secondary spinelet is largest. A few irregular rounded granules may occur on 

 the aboral portion of each plate. 



The actinal interradial areas are expansive, covered with a thin transparent membrane, 

 and with a compact plating of delicate imbricating scales. These scale-like intermediate 

 plates are more or less regularly hexagonal, and are arranged in columns parallel to the 

 median interradial line. The plates diminish in size and depth towards the margin, 

 where they become narrow elongate strips. Each plate bears two or three small rounded 

 granules irregularly disposed, and the large plates near the interior of the interradial 

 area have a few additional granules in proportion to their size. 



Colour in alcohol, greyish white, with a brownish or slightly orange shade over the 

 abactinal membrane. 



Young Phase. — There is a small specimen measuring R= 12 mm., r — 5 mm., which I 

 consider to be the young of this species. The example in question is especially note- 

 worthy on account of still possessing the embryonic plating on the disk. The plates, 

 though large, are very irregular, and I have not been able to reduce them to any formula 

 of arrangement ; the illustration on PI. XXVI. fig. 4 will give a fair idea of their form, 

 size, and position. 



