534 THE VOYAGE OF H.M.S. CHALLENGER. 
secondary or superficial mouth-spines, which are longer than 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°'1 Fahr. ; surface temperature 73°°5 Fahr. 
