500 THE VOYAGE OF H.M.S. CHALLENGER 



indeed so great is their development that nearly all individuality of fascicular character is 

 obliterated, and they appear to form a compact muscular tissue. In certain lines, how- 

 ever, aloug the sides of the rays there seems to be a tendency towards a greater and more 

 definite development of fibres in a lineal direction, parallel to the median line of the ray. 

 The spiracula are very few in number, quite microscopic, and occur in little groups of six 

 to ten which are widely and irregularly distributed. The oscular orifice is large, the 

 valves when closed forming a cone of small elevation ; and the spines which compose them 

 are somewhat irregular both in number and gradation in size. 



At the base of each valve is a rather close aggregation of paxillae whereby a more or 

 less complete aunulus is formed around the orifice ; and from each of the five groups in 

 question proceed a number of spinelets, which radiate outward from the centre and consti- 

 tute a conspicuous feature. 



The ambulacra! furrows are very wide, subpetaloid, tapering to a fine extremity, and 

 constricted slightly near the actinostome. The armature of the adambulacral plates con- 

 sists of only one spine to each plate, which is long, cylindrical, tapering to a fine point, 

 and placed on a small process projecting into the furrow. The aperture-papillas are elon- 

 gate, covered with a very widely expanded membrane, imparting an acumino-spatulate 

 form. 



The mouth-plates are long and thin, the pair having the appearance of being pressed 

 together laterally, projecting aborally in a prominent rounded keel, and sloping off 

 somewhat ploughshare-like towards the mouth. Two spinelets similar to the general 

 adambulacral armature, only perhaps rather shorter, stand on the superficies of each plate, 

 one on the sloping curve leading to the adoral margin, and the other aboral to this and 

 more outward in relation to the median suture-line of the plates. These spinelets appear 

 greatly modified both in character and position as compared with the usual robust 

 secondary spinelets of the genus. The mouth-spines are represented by two spinelets, 

 similar in all respects to the spines of the adambulacral armature, only rather shorter, 

 placed on the horizontal margin of each plate, and close up to the junction with the first 

 adambulacral plate. 



The actino-lateral spines are very long, the longest being about the fifteenth from the 

 mouth. On the inner part of the ray they are comparatively delicate when regard is had 

 to the size of the specimen, increasing, however, in robustness as they proceed along the 

 ray, those near the angle and the succeeding ones being strong and thick. The spines 

 reach quite up to the median line of the interradial area ; indeed in the outer half they 

 pass beyond and overlap, whilst the longest spines which fall in the interbrachial arc are 

 much longer than the distance between the median interradial line and the margin of the 

 furrow. In consequence a prominent outward peak is produced in the place of the angle, 

 the web being much contorted, and an abnormal growth not unfrequently takes place, 

 which produces an unsightly excrescence, as well as an irregular thickening of the tissue. 



