CHAP. IV.] THE VOYAGE ROME. 207 



smooth membrane between the arms — is covered with fascicles 

 of four to six diverging spines. These spines are about 3 mm. 

 in height ; and they support and stretch out a tolerably strong 

 membrane clear above the surface of the perisom, like the can- 

 vas of a marquee, leaving an open space beneath it. A close 

 approach to this arrangement occurs also in Pteraster. 



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

 ends at a large aperture, 15 mm. 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 marsupial tent ; a stout calcareous rod arises from the end 

 of the double chain of ossicles which form the floor of the am- 

 bulacral 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 be- 

 neath them open, these spines separate from one another, and, 

 like the ribs of a fan, spread out the membrane in a crescentic 

 form (Fig. 48) ; and when the valves close, the spines approxi- 

 mate and are drawn downward, the five valves forming together 

 a very regular, low, five-sided pyramid (Fig. 49). 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 par- 

 ticular specimen to which I have referred, which is considera- 

 bly 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 cen- 

 tral chamber, in which the eggs or the young had evidently 

 been lodged. I have on three occasions in species of the genus 



