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



If then there really were syzygial unions between the two outer radials and the first 

 two distichals and brachials respectively of Solanocrinus imperialis and Solanocrinus 

 gracilis, these two species would represent a type which is not as yet known to occur 

 in the recent Antedon at all, but only in Actinometra paucicirra and its allies ; while 

 the ten-armed Antedon costatus is represented at the present time by the various species 

 belonging to the type of Actinometra Solaris (PL LIII. figs. 2, 15). 



The three species of Antedon belonging to this first series which were dredged by the 

 Challenger, may be classified as follows : — 



Antedon, Series I. 



The two outer radials united by syzygy. 



A. Three distichals, the axillary a syzygy. 



I. Subsequent divisions of two articulated joints, . . . .1. fluctuans, n. sp. 



II. Subsequent divisions like the distichals. 



a. Three axillaries above the radials, . . . . .2. multiradiata, n. sp. 



b. Four axillaries above the radials, . . . . .3. microdiscus, Bell. 



1. Antedon fluctuans, n. sp. (PL VIII.). 

 Specific formula, A.R.3.2.(2).— -. 



Centro-dorsal a thick disk, bearing about twenty- five marginal cirri. These have 

 thirty to thirty -five joints, of which the fifth to eighth are much longer than broad. The 

 following ones diminish in length and gradually develop a sharp forward projecting 

 spine which decreases slightly in the short terminal joints, but increases again on the 

 penultimate as the opposing spine to a strong recurved claw. 



Three radials visible, the second free laterally, but united to the third by syzygy. 

 The rays are quite free and may divide four times. Three distichals, the axillary with a 

 syzygy. Palmars and post-palmars (when present) usually of two articulated joints. 

 Arms from twenty-six to nearly forty in number, and composed of short, smooth, and 

 obliquely quadrate joints. A syzygy in the third brachial ; the next anywhere between 

 the twelfth and the sixtieth, with others at intervals of six to twenty joints. 



The second distichal bears a long, tapering pinnule of about forty joints, the basal 

 ones tolerably stout and the terminal ones small. The second, and sometimes also the 

 third brachials have similar but smaller pinnules, and the following ones decrease slowly 

 in size, becoming long and slender again towards the arm-ends. 



Disk much incised, and the interradial regions more or less covered with rather large 

 plates. The ambulacra as far as the last axillary are raised and strongly plated ridges. 

 But those of the arms and pinnules, including even that on the second distichal, are 

 unprotected. 



