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



differences as do those of Antedon in the extent to which the radials appear on the out- 

 side of the calyx. In Actinometra maculata, Actinometra lineata, and Actinometra 

 stelligera the oentro-dorsal is so large that it actually supports the proximal ends of the 

 second radials, and nothing but the angles of the first radials can be seen externally 

 (PI. V. figs. 1, 2, 5, a, 6). Small portions of their sides can be seen in Actinometra Solaris 

 (PL V. figs. 4, 6, c) ; while in Actinometra paucicirra and in all the Phanogenia-\ike 

 forms the centro-dorsal only occupies a comparatively small space in the centre of the 

 radial pentagon, a considerable portion of which appears externally as seen in PL V. fig. 3c, 

 and in Pis. LIV, LXV. 



Not only the centro-dorsal, but also the radials of Actinometra present very 

 considerable differences from the corresponding parts of the Antedon-c&lyx, though 

 these differences are less distinct in the fossil than in the recent forms of both genera. 

 The outer or articular faces of the radials in Antedon are always much inclined to 

 the vertical axis of the calyx, and are usually much wider at their dorsal than at their 

 ventral ends, so that their outline is trapezoidal (PL I. figs. 6a, 8a ; PL II. figs. 1-5, a ; 

 PL III. figs, la, 46, 5a, 6a 7 ; PL IV. figs. 2a, 3a). Antedon carinata and Antedon 

 macronema are, however, somewhat exceptional in this respect, the width of their 

 articular faces being very much more uniform ; and they further differ from most species 

 of Antedon and resemble Actinometra (PL V. figs. 1-5, o) in the relatively great diameter 

 of the central funnel of the calyx (PL III. fig. id ; PL IV. fig. 3b). For as a general 

 rule the opening of the central funnel which is bounded by the upper edges of the radials 

 is very narrow, their ventral surfaces being quite small and having a steep inward slope. 

 Hence when the calyx is viewed from above the greater part or even the whole of these 

 inclined external faces is visible, always down to the opening of the central canal in the 

 transverse articular ridge (PL II. fig. Ad ; PL III. fig. 6c); while sometimes even the fossae 

 for the attachment of the dorsal ligament are visible in a superior view (PL I. figs. 6, 8, b; 

 PL II. figs. 1-3, 5, d; PL III. fig. 4a). This last is especially the case in Antedon macronema 

 (PL IV. fig. 3b), though the reverse is true of Antedon carinata (PL III. fig. id). 



Most species of Antedon have large muscle-plates, which greatly increase the height of 

 the distal faces of the radials (PL I. figs. 6, 8, a ; PL II. figs. 1-5, a ; PL III. figs. 46, 5a, 

 6c/). They are fairly large in Antedon carinata (PL III. fig. la), but in Antedon 

 macronema they are quite small and linear and barely distinguishable in side view from the 

 pair of fossae immediately below them, though they are seen rnoi*e clearly when viewed from 

 above (PL IV. figs. 3o, b). This lower pair of fossae was described by Dr. Carpenter 1 as 

 serving for the attachment of the interarticular ligaments. The ridges which generally 

 separate them from the upper fossae mostly start from the raised rim round the opening 

 of the central canal and run more or less obliquely outwards to meet the sides of the 

 radials (PL II. figs. 2-5, a ; PL III. figs. 46, 5a, 6d). In Antedon antarctica, however, 



1 Phil. Trans., 1866, p. 714. 



