2o8 DISCOVERY REPORTS 



portion is abruptly narrower than the genital, and devoid of plating except for the 

 regular lateral, spaced costae, which, except for the first i or 2 beyond the genital region, 

 do not reach entirely across the ray but simply support the thin integument as props 

 extending upward from, usually, each fourth adambulacral. These lateral " ribs" carry a 

 vertical series of 3 acicular long spines enveloped in an ovoid fleshy sacculus covered 

 with minute pedicellariae. These thick sacculi are found also on some of the abactinal 

 spines of the genital region, and possibly in living specimens are still more numerous 

 but have been lost by abrasion in the dredge. 



The disk is dorsally convex, the convexity accentuated by the dorso-ventrally con- 

 stricted ray at its opening into disk (PI. XVI, fig. 4). Its abactinal wall is reinforced by 

 irregularly roundish or short-lobed, slightly imbricating plates 0-5-1 mm. in diameter 

 and carrying i, 2, or 3 skin-covered spinelets, subacicular or variously pronged. Some 

 examples are given in accompanying figures. Apparently the disk spinelets never carry 

 the clavate sacculus. Scattered large papulae are more numerous in a marginal zone than 

 at centre of disk. Pronged spinelets are more numerous in immature specimens than 

 in fully adult ; that is, later additional spinelets are more likely to be of the acicular type. 



The abactinal skeleton of the genital region, like that of disk, is normally obscured by 

 the skin and is more open, with very irregular elements. A study of growth stages does 

 not indicate that the abactinal plates of the genital region, in excess of those of the 

 simple arches, represent ancestral arches partly suppressed. On the abactinal surface 

 between arches II-III, and III-IV the extra plates have, in medium-sized specimens, an 

 easily discernible transverse alignment (PI. XVI, fig. 3). On the proximal part of the 

 genital region the larger plates have 2 or 3 lobes and carry 1-3 sharp acicular spines 

 while the oblong connectives which join them and achieve thus a very irregular reti- 

 culum carry usually a shorter one. On the sides of this region the arches of plates are 

 arranged, with slight variations as in PI. XVI, fig. 3, which represents a medium-sized 

 male with gonopore in the middle of the large intercostal space I. This area of thin skin 

 becomes, in the female, one side of the nidamental cavity (PI. XVII, fig.i). This area is 

 always bounded proximally by arch I, the lower end of which is formed by inferomarginal 



2 resting on adambulacrals 2 and 3. Inferomarginal i and adambulacral i remain on the 

 disk when a ray is detached. When the nidamental cavity is functioning, arch II is not 

 so straight as shown in PI. XVI, fig. 3, but is strongly bowed or bent distally. The oviduct 

 opens at about mid-height on its proximal border. Arches III and IV extend completely 

 across the ray from side to side; but arch V is represented on either side by a " rib" of 



3 or 4 plates, each bearing a spine. From arch V or VI onwards, each rib of plates consists of 

 an inferomarginal, a superomarginal, and a dorsolateral plate, each carrying a long spine 

 covered by a fleshy sacculus (PI. XVI, figs, i, 2). Toward end of ray there may be only 2 

 lateral plates and spines. In the type there are 4 complete arches plus 19 pairs of lateral 

 ribs. 



Small isolated radial plates occur sporadically along the radial line and carry groups of 

 pedicellariae sometimes without a central spinelet. 



The terminal plate, though small, is modified to protect the relatively large terminal 



