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



that the genus Phanogenia was instituted by Loven 1 for the reception of species 

 presenting these characters. 



It was pointed out in my preliminary report, 2 however, that the stellate appearance 

 of the centro-dorsal in Phanogenia (PI. LVII. fig. 1) "appears to be one of the concluding 

 stages of a long series of changes in the shape and relations of the centro-dorsal, which 

 do not commence until some time after the loss of the stem and the entry upon the 

 free state of existence." The earlier stages of these modifications are well shown in a 

 series of specimens of Actinometra 'paucicirra, which is very abundant at Cape York 

 (PL LIV.). In the youngest individual of the series the centro-dorsal is a thin 

 and slightly convex circular disc, about 2 mm. in diameter, which bears five pairs of cirri, 

 one pair opposite each interradius. They reach 6 mm. in length and consist of about 

 fifteen joints, which are tolerably mature in their general characters (PI. LIV. fig. 10); 

 the next stage is a slightly older individual in which all the cirri have fallen away from 

 the centro-dorsal and the obliteration of their sockets has commenced (fig. 9). This 

 process has been carried further in the larger and more distinctly pentagonal centro- 

 dorsal shown in fig. 8, though it has gone on rather unequally, some of the sockets 

 being much more obliterated than others. 



Scarcely any trace of sockets can be made out in the original of fig. 5, but the centro- 

 dorsal is a thin pentagonal disc with the appearance of processes at some of its angles, 

 which are more probably, however, the ends of the basal rays. Its surface is much more 

 nearly flush with that of the radials in the full-grown specimen shown in fig. 2, still, 

 however, retaining its pentagonal shape. Fig. 1 shows another modification, each 

 angle of the pentagon being marked by a more or less deeply impressed pit in which the 

 basal ray is sometimes visible. The form represented in fig. 3 has a more rounded 

 centro-dorsal, which is flush with the radials at its edges, and shows the basal rays at its 

 angles; while there are indications of pits at the distal angles of the sutures between the 

 first radials. The sides of the centro-dorsal in this specimen are slightly concave, and 

 this character is much more distinct in figs. 6, 7, so that the shape becomes markedly 

 stellate. In the former the centro-dorsal (as viewed from the dorsal side) is above the 

 level of the radial pentagon; but in the latter it is relatively much lower, so that its 

 surface is flush with that of the radials, the proximal edges of which are convex in 

 correspondence with the stellate outline of the centro-dorsal. Fig. 4 shows a similar 

 case in which the centro-dorsal is pentagonal. The effect of its complete withdrawal 

 into the radial pentagon is to make it entirely invisible in a side view of the calyx, as 

 seen in PL V. fig. 3 b ; while the dorsal surface of the united radials becomes very 

 deeply hollowed for its reception (PL V. fig. 3c) instead of being slightly convex, as i3 

 more usually the case (PL V. figs, lc, 5d). 



1 Phanogenia, ett hittils okandt slagte af fria Crinoideer, Ofvcrsigt k. Vetensk.-Akad. ForhandL, 1866, p. 231. 

 *Proc. Roy. Soc, 1879, vol. xxviti. p. 390. 



