466 TITE VOYAGE OF H.M.S. CHALLENGER. 



Form stellate. Rays rather short and broad, the length from the interbrachial arc 

 being about equal to the diameter of the disk. The interbrachial arcs are acute, and 

 the rays are broad at their base and taper only slightly as they proceed outward until 

 close to the extremity, where they taper abruptly and rapidly to a pointed tip, which is 

 turned upward. This recurvature of the tip causes the rays to have a more or less obtuse 

 appearance when casually viewed from above, and the character is further emphasised 

 by the slight degree of tapering along the greater portion of their length. The breadth of 

 the ray is greater than the height ; and the abactinal surface is convex, uniting with an 

 angular margin to a plane actinal surface. A transverse section of the ray would thus 

 present a regular plano-convex outline. 



The disk is well developed, subdepressed, convex, and slightly inflated, its height not 

 much greater than that of the rays at the base. The abactinal skeleton of the disk and 

 rays alike consists of cruciform ossicles, in the shape of a St. Andrew's cross, with delicate 

 prolongations, or supplementary trabeculse, the extremities of which impinge on the cor- 

 responding extremities of adjacent ossicles ; the whole forming a very regular network over 

 the entire surface. On the centre of each of these ossicles is borne, on a little boss, a 

 fascicule of three to six moderately elongate delicate spinelets, of equal length, which radiate 

 apart very slightly. The spinelets are each enveloped in a rather thick membranous sheath, 

 and the sheaths of the respective spinelets are united in the interior part of the fascicule, 

 so that, although the sheathed spinelets have the superficial appearance of maintaining their 

 independence, they are in reality bound together, and the spinelets constituting a fascicule 

 are in consequence probably capable of but very limited expansive movement. The fasci- 

 cules of spinelets are isolated and tolerably spaced; and the length of the spinelets diminishes 

 as they proceed along the ray ; the spinelets, however, increase in length as they approach 

 the margin of the ray, and those of the series at the extreme margin are stout and robust, 

 with usually four spinelets in each fascicule. In the interspaces^ or meshes of the cal- 

 careous network are a number of small, vermiform, almost thread-like papulae, from 

 three to six in each. 



The armature of the adambulacral plates forms a continuous series with that of the 

 plating which extends up to the margin of the ray. Six isolated spines form a single 

 transverse row between the furrow and the marginal series of fascicules ; and each is 

 articulated upon a small rounded boss or tubercle. The innermost spine is very small 

 and situated quite within the furrow ; the next is much larger, whilst the third and suc- 

 ceeding spinelets are longer, and are the largest on the test. Each of these spinelets is 

 enclosed in a membranous sheath, which, in the case of the two nearest the furrow, has 

 elongate saccular prolongations, that of the small inner spinelet being thin and threadlike. 

 The sheaths of the outer spines are stouter and more fleshy in appearance, and are little 

 if at all prolonged. The intermediate space between the spinelets is so thickly covered 

 with membrane that even after removing a few of the spines, I am unable to say whether 



