236 MB. W. PERCY SLADEN ON THE 



The interradial angles appear to have been well rounded, with the 

 lesser radius probably in the proportion of about 57 per cent. ; 

 JB=35 millim., r=about 20 ; but the specimen is unfortunately 

 so much damaged in each of the arm-angles that it is impossible 

 to give the smaller dimension exactly. Eadii well produced, fine 

 and tapering at the extremities. Dorsally the centre of the disk is 

 elevated into a sharp conoid, and the radii are prominently arched. 



Supradorsal membrane very delicate. Spinelets of the paxiUae 

 prominently projecting, very delicate, and tapering towards their 

 extremity, distributed regularly at uniform intervals apart over 

 the whole dorsal area. Numerous very fine thread-like fibrous 

 bands pass between the tips of the spinelets, crossing over and 

 under one another, but not merging or forming a coherent reti- 

 culated structure. The fibres are not tightly stretched between 

 the tips of the spinelets, but slope downward at a high angle, like 

 slackened ropes round a tent-pole ; in consequence of this and 

 of their great prominence, the spinelets appear to superficial exa- 

 mination to stand like well-spaced conical prickles upon the dorsal 

 area. The oscular orifice is of moderate size, the outer margin 

 (from which the valves take their rise) being marked off" by pro- 

 minent sharp spinelets into a pentagon, 9'5 millim. in diameter, 

 the angles opposite to the radii. The valves are very regularly 

 subtriangular, composed of 10 to 12 radiating spines, and when 

 closed form a pyramidal peak in the centre of the disk. 



Ambulacral furrows narrow and deeply sunken, constricted 

 near the mouth, widest about the outer third, and then sharply 

 tapering to the extremity. Ambulacral spinelets 3, short, stout, 

 tapering, compressed, placed in line oblique to the direction 

 of the furrow and also to the horizontal plane of the ray. The 

 ambulacral spines are quite hidden in the furrow, not webbed 

 together, but probably invested with a rather long membrane. 

 Aperture-papillae large and squarely oval. 



Mouth-plates somewhat broad, short, almost perpendicular in 

 position ; keel flattened. Each plate bears two short thick secon- 

 dary superficial spinelets, the aboral one most robust. One 

 mouth-spine stands above the innermost secondary ; and another, 

 much smaller, is placed isolately on the horizontal lateral margin 

 of the plate. 



Actino-lateral spines 27 -|- , perhaps about 30, robust at the extre- 

 mity of attachment, but very delicate and taperin outward, not 

 meetiug in the intei*radium. 



