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



may be ; but as they occur in four species of Actinometra, one American and 

 three from the Eastern Archipelago, and are unknown in Antedon, they provide us 

 with another potential character of the former genus which has a certain systematic 

 value. 



The presence of a terminal comb on the lower pinnules is, however, an absolutely 

 constant character of Actinometra. It varies much in its development (PI. LIII. 

 figs. 3-6 ; PI. LVI. figs. 2, 4 ; PL LXI. figs. 8-10 ; PL LXIII. figs. 5, 7 ; PL LXVI. 

 figs. 3, 5 ; PL LXVII. figs. 2, 4 ; PL LXVIII. fig. 3), but it is always present ; and this 

 peculiarity, together with the invariable absence of sacculi on the ventral perisome, 

 enables single arms of Actinometra to be recognised with the utmost certainty. 



The arms and pinnules of this genus are never provided with the ambulacra! skeleton 

 which is so well developed in many species of Antedon; and the character which is so 

 often associated with this, viz., the lateral flattening of the lower parts of the rays, is also 

 entirely absent in Actinometra. This indeed is only to be expected, for the three groups 

 of A ntedo ?i-species which present these combined characters are almost entirely hmited 

 to the abyssal and continental regions, while Actinometra is essentially a shallow-water 

 genus, having only been obtained nine times at depths exceeding 200 fathoms. 



In certain localities, however, e.g., Cape York and Port Curtis in Queensland, species 

 of Actinometra occur with the disk very completely plated, although it may be entirely 

 membranous in the same species elsewhere. This is especially noteworthy in the 

 cases of Actinometra Solaris, Actinometra pectinata, and Actinometra paucicirra ; but 

 however well plated the disk may be, there is no ambulacral skeleton on the arms and 

 pinnules, any more than there is in those species of Antedon like Antedon elegans and 

 Antedon multiradiata, which have the two outer radials united by syzygy and a thickly 

 plated disk (PL IX. fig. 2 ; Part I., pi. lv. figs. 3, 4). The essential characters of the 

 radials of Actinometra have been fully explained on pp. 24-26, and need not therefore 

 be further discussed. 



The centro-dorsal is very often only a thin flattened disk, with an imperfect double 

 row of cirrus-sockets round its margin (PL IV. fig. 4a; PL V. figs, lb, id, 2b, 2d, 2e, 2d; 

 PL LII. figs. 1, 2 ; PL LIII. fig. 1, 2, 15 ; PL LXII. figs. 1, 2 ; PL LXIV. figs. 1, 3) 

 There are not often more than about twenty functional cirri on the centro-dorsal at the 

 same time ; but this number is sometimes exceeded (PL LX. figs. 1-3 ; PL LXVI. fig. 4). 

 On the other hand, the centro-dorsal is occasionally reduced to the condition of a mere flat 

 plate without any trace of cirrus-sockets (PL LIV. figs. 1-8), and it is often separated 

 from the radial pentagon by more or less definite slits (PL LVII. fig. 1 ; PL LXI. fig. 1 ; 

 PL LXIII. fig. 6; PL LXV. figs. 1, 5, 6; PL LXVII. fig. 1). It has been pointed out above 

 that the new genus Phanogenia was established by Lov^n for a species of Actinometra 

 possessing these characters ; and the nature of the change which produces them has 

 already been noticed on pp. 13-16. It need not therefore be further considered here, 



