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



accordingly as Antedon fluctuans. It is curious, however, that while the two most 

 dissimilar forms were dredged at the same Station in the Arafura Sea, the intermediate one 

 was obtained at a much less depth in Torres Strait, and that no others have been met with 

 at any intervening locality. When examining the " Alert " specimen I found a small 

 Myzostoma upon it, which Professor von Graff has named Myzostoma quadricaudatum. 



Antedon fluctuans is a type of considerable interest from a systematic point of view. 

 For the syzygial union of the two outer radials is in no case accompanied by a similar 

 union of the first two joints after each subsequent axillary. Whether there be three 

 distichals, as is normally the case, or two only, as in some exceptional rays, there is 

 always the bifascial articulation between the first two joints above the radials and above 

 every successive axillary, just as in the ordinary many-armed Antedons. In the case of 

 Antedon fluctuans, the palmar and post-palmar series (when present) normally consist of 

 two joints only, and this character distinguishes the type from Antedon midtiradiata 

 and Antedon microdiscus, in which there are three joints between the successive arm- 

 divisions (PI. IX.; PL XXXVII. fig. 3). I know of no other described species but 

 these with which Antedon fluctuans is likely to be confounded, provided, of course, that 

 the syzygial union of the radials be properly recognised. 



2. Antedon midtiradiata, n. sp. (PI. IX.; Part I. pi. lv. figs. 3, 4). 



Specific formula, A. R.3.3.3.— . 



Centro-dorsal a thick, slightly convex disk, bearing from twenty to twenty-five 

 marginal cirri. These are rather long, consisting of forty or fifty, or occasionally more 

 joints, few or none of which are longer than wide. The last half have a small blunt 

 spine projecting slightly forwards, which forms a strong and sharp opposing spine on the 

 penultimate. 



Three radials visible, the second short aud free laterally, but united to the third by 

 syzygy. The rays are quite free and may divide four times, each series of three joints 

 with the axillary a syzygy. About forty arms of smooth and short triangular joints, 

 which become blunter and more square towards the ends. A syzygy in the third 

 brachial, the next anywhere between the sixteenth and forty-fifth joints, with others at 

 intervals of seven to nineteen joints. 



The distichal pinnules of moderate length, consisting of about twenty-five stout 

 joints ; the size gradually decreases to that on the second brachial, and the next pair are 

 considerably smaller, the following ones increasing slowly in size, but never becoming 

 very large. 



Disk much incised and paved with large plates between the ambulacra, which are 

 elevated ridges with plated walls, but the plating scarcely extends beyond the level of the 

 last axillary. 



