PEOPAftATIOIir or CEETAIIT ECHIIfODEEMS. 75 



that the angles between the arms are entirely filled up by a fleshy 

 lamina stretched over and supported by spines, the body thus 

 becoming a regular pentagon. The arms are 55 to 60 raillims. in 

 diameter ; the upper surface of the body, disk, and arms, all the 

 surface, except the smooth membrane between the arms, is covered 

 with fascicles of four to six diverging spines. These spines are 

 about 3 millims. in height ; and they support and stretch out a 

 tolerably strong- membrane clear above the surface of theperisome 

 like the canvass of a marquee, leaving an open space beneath it. 

 A close approach to this arrangement occurs also in Fteraster. 



At the apical pole the upper free membrane runs up to and 

 ends at a large aperture, 15 millims. in diameter, surrounded by a 

 ring of five very beautifully formed valves. These valves do not 

 essentially differ from the ordinary radiating supports of the mar- 

 supial tent ; a stout calcareous rod arises from the end of the 

 double chain of ossicles which form the floor of the ambulacra! 

 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. 11) ; and when 

 the valves close, the spines approximate and are drawn down- 

 wards, the five valves forming together a very regular, low, five- 

 sided pyramid (fig. 12). Looking down into the chamber when 

 the valves are raised, the vent is seen on a small projecting pa- 

 pilla in the centre of the floor ; and between the supporting ossicles 

 of the valves, five dark open arches lead into the spaces opposite 

 the reentering 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 remained on the floor 

 of the central chamber, in which the eggs or the young had evi- 

 dently been lodged. I have on three occasions found the eggs 

 beneath the membrane in the angles of the arms and, in a more 

 advanced stage, congregated in the central tent, but never under 

 circumstances such that I could keep and examine them ; exposed 



