REPORT ON THE CRINOIDEA. 13 



figs. 1-5, 6). Thus then the interradial symmetry of the centro-dorsal is an altogether 

 secondary condition, and is due not to the possible presence of radially situated under- 

 basals, as supposed by Wachsmuth and Springer, but to the fact that the radials 

 themselves rest upon the plate, the primary radial symmetry of which becomes 

 altogether obscured when it begins to increase in diameter and to develop cirri, 

 coincidently with the retromorphosis of the basals. 



The external form of the centro-dorsal varies very greatly among different species of 

 Comatulge. It is very distinctly conical in Atelecrinus (PI. VI. figs. 5, 7). In the 

 three chief of the remaining endocyclic genera (Antedon, Eudiocrinus, and Promacho- 

 crinus) it is occasionally somewhat hemispherical or subcorneal, with the cirrus-sockets 

 arranged rather irregularly (PI. I. figs, la, 6a, 8a; PI. II. figs. 1-3, a; PL III. figs. 46, 

 5a, la; PI. XXX. figs. 1, 2, 4); but in some cases, as in Antedon quinquecostata, it is 

 more distinctly pentagonal and columnar, with the sockets grouped in alternating rows 

 (PI. III. fig. 6d), while in Antedon balanoides it is distinctly conical (PI. XXXIII. 

 fig. 6). In other forms again the dorsal pole is flattened (PI. II. fig. 4a), and this is 

 especially the case in Antedon carinata and Antedon macronema (PI. III. figs, la, 36 ; 

 PI. IV. fig. 3a), which in this character, as in some others, exhibit a variation in the 

 direction of Actinometra. On the other hand, Antedon quinduplicava and Antedon 

 disciformis, which are still more like Actinometra in the small number of functional 

 cirrus-sockets and in the discoidal shape of the centro-dorsal, belong unmistakeably to the 

 genus Antedon in the relative height of the radials (PL IV. figs, la, 2a). 



In most species of Actinometra the centro-dorsal is a thin flattened disc, often with 

 only one row of functional cirrus-sockets (PL IV. fig. 4a ; PL V. figs. 16, Id, 26, 2a", 

 2e, 46) ; though in Actinometra stelligera it is thicker and bears a comparatively large 

 number of sockets (PL V. figs. 56, 5c). 



As a general rule the shape of the centro-dorsal is tolerably constant in any individual 

 species of Antedon, being hemispherical in Antedon eschrichti (PL I. fig. 8a; PI. 

 XXIV. figs. 10, 11), columnar in Antedon quinquecostata (PL III. fig. 6d), and more 

 discoidal in Antedon carinata (PL III. figs, la, 36). But in Antedon phalangium it 

 exhibits a very considerable amount of variation, being hemispherical in some forms, but 

 greatly elongated and conical in others (PL XXVIII. fig. 2). 



In some species of Actinometra the obliteration of the cirrus-sockets on the centro- 

 dorsal is carried to a very much greater extent than in Antedon ; and the number of 

 functional sockets, which is at no time large, is often extremely small. In some types the 

 changes in the centro-dorsal do not stop here, but it is reduced to the condition of a flat 

 pentagonal plate within the ring of radials as in Actinometra paucicirra (PL LIV. 

 figs. 1-7) ; while in species like Actinometra typica (PL LVII. fig. 1), the sides of 

 this plate undergo resorption, so that clefts appear between it and the radials. This 

 gives the base of the calyx an appearance so different from that of the ordinary Comatulas 



