238 THE VOYAGE OF H.M.S. CHALLENGER. 



laterally, developing a bluntly pointed keel, which passes into the dorsal spine of the 

 penultimate. 



The angles of the first radials just visible ; the second short and partly united 

 laterally ; axillaries wide, more than twice their length, and almost triangular. The 

 rays may divide three times, each division of two joints, the axillary without a syzygy. 

 The first few joints above the radial axillary on the outer side of the ray have their 

 outer edges curved and folded ; while the lower brachials, both of the inner and of the 

 outer arms, have their apposed sides flattened against one another. Twenty-seven long and 

 tapering arms, of about one hundred and eighty joints, the lower ones discoidal and their 

 successors shortly triangular, becoming more quadrate in the middle, and in the terminal 

 third more nearly square, elongating slightly towards the end. A syzygy in the third 

 brachial ; the next between the fifteenth and eighteenth, with others at intervals of 

 eight to eighteen joints. 



The first pinnule on the outer side of the ray may reach 8 mm., with twenty-seven 

 joints, but on the inner arms it is generally somewhat smaller. That of the third brachial 

 is about equal to it. The second pinnule is also rather larger on the outer than on the 

 inner arms, reaching 1 5 mm., with about thirty joints, of which the first third are moderately 

 stout, and the remainder more slender and somewhat elongated. The pinnules of the 

 next three brachials (fifth to seventh) are of nearly equal size, but the fourth pair are only 

 about half their length, with fifteen joints, and the next pair are still smaller. 



Disk naked and much incised ; sacculi abundant at the sides of the pinnule-ambulacra. 



Colour in spirit, — dark purple, with greenish-white spots on the disk. 



Disk 20 mm.; spread about 30 cm. 



Locality. — Tongatabu reefs. One specimen. 



Remarks. — This fine specimen is not unlike Antedon articulata, Muller, but has a 

 smaller number of cirrus-joints, with less well defined spines than occur in that species. 

 In fact the spines are hardly anything more than a small pointed process in the middle of 

 the sharp dorsal keel. The fourth pinnule is relatively smaller and the second syzygy 

 nearer the disk than in the type of Antedon articulata ; and there are less than thirty 

 arms, instead of nearly forty, or even more, as palmar axillaries are not always developed, 

 and there are no post-palm ars at all. 



Antedon, Series IV. 



Three distichals, the first two articulated, and the third axillary with a syzygy. 



Remarks. — The tridistichate species of Antedon are less numerous than those in 

 which only two distichals are present, but the two series have many points of 

 resemblance, both in their distribution and in their modifications of structure. 



