NARRATIVE OF THE CRUISE. 



393 



valves do not essentially differ from the ordinary radiating supports of the marsupial 

 tent ; a stout calcareous rod arises from the end of the douhle chain of ossicles which 

 form the floor of the ambulacral groove. From the outer aspect of this support three or 

 four spines diverge in the ordinary way under the tent-cover; but from its inner aspect 

 six or eight slender spines rise in one plane with a special membrane stretched between 

 them. When the valves are raised and the pentagonal chamber beneath them open, these 

 spines separate from one another, and, like the ribs of a fan, spread out the membrane in 

 a crescentic form (fig. 148) ; and when the valves close, the spines approximate and are 



Fig. 148. — Hymenaster nobilis, Wyv. Thorns. Southern Ocean. Half the natural size. 



drawn downwards, the five valves forming together a very regular, low, five-sided pyramid 

 (fig. 149). Looking down into the chamber when the valves are raised, the vent is seen 

 on a small projecting papilla in the centre of the floor ; and between the supporting 

 ossicles of the valves, five dark open arches lead into the spaces opposite the re-entering 

 angles of the arms, which receive the ducts of the ovaries. In the particular specimen 

 to which I have referred, which is considerably the largest of the genus which we have yet 

 met with, there were one or two eggs in the pouch, but they were apparently abortive. 

 It seemed that the brood had been lately discharged ; for some oval depressions still 



(naee. chall. exp. — vol. i. — 1884.) 



50 



