534 THE VOYAGE OF H.M.S. CHALLENGER 



secondary or superficial mouth-spines, which are longer thau the mouth-spines proper ; 

 the most aboral is the longer and stands at about one-third of the distance from the 

 adoral extremity of the plate ; the second is smaller and is placed close behind the 

 marginal series of mouth-spines. All these spinelets, both mouth-spines proper and 

 secondaries, are covered with membranous sheaths, which in the case of the large aboral 

 secondary spines are widely expanded at the base. The mouth-spines are united by a 

 membranous web, but there is no connection between the armature of the two united 

 plates. 



The actinostome is large, occupying nearly two-fifths of the actinal surface of the 

 disk, the mouth-plates consequently could not be apposed, and the mouth is situated in 

 the centre of a naked leathery membrane, and is furnished with a well-developed muscular 

 lip, the delicate and regular plications of which give a very elegant appearance to this 

 structure. Near the margin of the actinostome the membrane is semitransparent, and 

 the internal viscera may be indistinctly traced through it. 



The dorso-central valves when closed form as a whole an externally subcircular button 

 from 6 to 7 mm. in diameter. The valves are triangular and composed of delicate spine- 

 lets, a dozen or more in each, united together by membranous web. The spinelets decrease 

 in length regularly from the central ones in each fan-like valve ; and the membrane is 

 gracefully festooned between the extremities of the spines. Externally there is an abrupt 

 flexure of the fan at its base, which gives the button-like character to the apparatus 

 already noticed. Some vesicular protuberances are visible through the spaces left be- 

 tween adjacent valves, but I am unable, without an undesirable mutilation of the specimen, 

 to offer any positive observation as to whether these valves open into a definite infra- 

 dermal cavity ; my impression is that such a cavity does not exist in this form. A pro- 

 tuberant anal membrane appears to be traceable. 



The madreporiform body is moderately large and situated in the interradial sulcus, 

 external but close to the dorso-central valves ; the striations are numerous but rather 

 coarse, and radiate with considerable regularity from the centre. 



The ambulacral tube-feet are large and regularly biserial. They are furnished with a 

 large fleshy sucker-disk, which, when viewed laterally, is seen to be of greater diameter 

 than the immediately adjoining part of the tube, and from which it is marked off by a 

 well-defined constriction. The sucker-disk has consequently the character of a button- 

 like extremity ; and in the centre there is a conspicuous invaginated depression. 



Colour in alcohol, a uniform yellowish ashy grey. 



Locality. — Station 323. Off the coast of South America, east of Buenos Ayres. 

 February 28, 1876. Lat. 35° 39' 0" S., long. 50° 47' 0" W. Depth 1900 fathoms. 

 Blue mud. Bottom temperature 33° - l Fahr. ; surface temperature 73° - 5 Fahr. 



