8 TEE VOYAGE OF H.M.S. CHALLENGER. 



less than half their length ; and the upper margin only of the plate forms the boundary 

 of the abactinal surface of the ray. Ou the centre of each plate is a large well-defined 

 tubercle, on which is articulated a robust, cylindrical, tapering spine, the ninth from the 

 interradial line being about 8 mm. in length. The surface of the plate is covered with 

 membrane and bears no spines, excepting occasionally a minute thorulet, irregularly 

 placed near the base of the large spine. 



In the median interradial line there is one large odd supero-marginal plate, developed 

 abactinally into a prominent, truncate, conical tubercle, upon which is borne a powerful 

 robust spine about 8 mm. in length, directed vertically, and thicker than any of the other 

 spines on this species. 



The infero-marginal plates are similar in form and character to the superior series, 

 each being nearly exactly beneath its corresponding upper companion. Like them, each 

 has a prominent tubercular eminence, upon which is borne a straight, robust, tapering, 

 and sharply-pointed spine, the sixth or seventh from the interradial line measuring about 

 10 mm. in length ; the length decreasing slightly as they proceed along the ray. A 

 second similar but rather smaller spine, articulated on a tubercle, stands close to the 

 lateral spine on the inner side. The surface of the plate is covered with membrane ; 

 and two or three irregularly placed microscopic thornlets may be present. On the two or 

 three innermost plates on each side of the median interradial line, the lateral spines 

 and their companions are greatly reduced in size, being little more than mere miliary 

 spinelets. 



In the interbrachial arcs, the marginal plates have the appearance of being brought 

 over upon the disk ; and their surface forms a peculiar bevelled area sloping outwards 

 and downwards, the lower margin only of the infero-marginal plates falliug in the outline 

 of the interbrachial arc. 



The adambulacral plates are large and massive, of great breadth, and rather broader 

 than long. They are comparatively widely separate, and the interspaces are filled up with 

 ligament; the furrow margin is slightly convex. Their armature consists of: — (l.) a 

 furrow series of five very small, short, equal, cylindrical, obtusely pointed spinelets, which 

 radiate apart and form a small isolated semicircular comb directed over the furrow, the 

 successive combs being well spaced apart. (2.) On the actinal surface of the plate are 

 two large, robust, tapering, but obtusely pointed spinelets, placed one behind the other, 

 which are subequal in length, or occasionally the outermost is the longest ; this measures 

 4 mm. on the sixth or eighth adambulacral plate, the longest spinelets of the furrow 

 series not exceeding 1 mm. One minute thorulet usually stands on the adoral side of the 

 outer of the two superficial spines ; no other spinelets are present, and the surface of the 

 plate is covered with a membranous tissue. The ambulacral furrows are comparatively 

 widely open. The tube-feet are large, and have a rounded, well-developed, knob-fike 

 termination. 



