RETORT ON THE ASTEROIDEA. 619 



terminates. The membrane is furnished with elongate saccular bands placed saddle-like 

 upon the truncately arched abactinal surface of the ray with great regularity, one on 

 each side of each pair of ossicles, limited, however, to the sides, and not united across 

 the median line. The sacculi extend from the horizontal line of the ambulacral " verte- 

 brae " to the margin of the ray, this dimension being about 25 mm., and the breadth of 

 the band is about 0*75 to 1 mm. They have their surface crowded with very minute 

 pedicellariaa, visible only under high magnification. These are similar to those already 

 mentioned, but are smaller, their length being about O'll mm. 



The section of the ray midway between the centre and the extremity gives an outline 

 which may be described as subtriangular with the apex truncate, the greater portion of 

 the base-line is occupied by the ambulacral furrow, the extremities of the line being 

 represented by the adambulacral plates. 



The angular margin of the ray is fringed by a continuous series of elongate lateral spines ; 

 these are attached to a rudimentary marginal plate ankylosed to the outer side of the 

 adambulacral plates, and are directed horizontally and outward at an angle of less than 45° 

 to the axis of the ray. They are usually borne on every other adambulacral plate, but 

 occasionally they are present on two consecutive plates. Their presence is very irregular 

 on the basal part of the ray (for about 25 mm.), and they are quite short and stunted (3 - 5 

 mm. in length) until the ovarial region is passed ; they then increase in length, and con- 

 tinue to increase gradually to a slight degree, their greatest length being near the middle 

 of the ray, where they measure about 12 mm., and then after some distance gradually 

 diminish in length towards the extremity. The spines are delicate and cylindrical, about 

 0"26 to 0"3 mm. in thickness near the base, and taper throughout to an extremely fine, sharp 

 point. The base forms a small condyle-like head, by which the spine is articulated to its 

 plate. The spine is encased in a delicate membranous sheath, the surface of which is 

 crowded with minute pedicelhuise, so small as to be quite invisible to the naked eye, and 

 only giving the appearance of a delicate superficial asperity on the spinelet when examined 

 with a hand-magnifier. The sheath is prolonged for a short distance beyond the extremity 

 of the spine in the form of a slightly bulbous sac, which is usually turned a little to one 

 side as if the spinelet maintained an attachment to one of the lateral walls of the sacculus 

 rather than occupying a central position ; furthermore, the membranous sheath does not 

 taper in conformity with the tapering of the contained spine (see fig. 4). The pedicellaria? 

 are of the same kind as those already noticed ; they diminish a little in size as they 

 approach the extremity of the spine, and the largest, which are situated near the base, 

 measure about 0*1 to 0*116 of a millimetre in length. 



The ambulacral furrow occupies nearly the whole of the actinal surface of the ray, being 

 2 "5 mm. in width at a place where the whole ray is 575 mm. The adambulacral plates 

 form the rest of the area, and have the appearance of constituting only a thin margin 

 to the furrow ; their shape as viewed from beneath is semicylindrical, with the furrow- 



