I 



THE BRITISH SECONDARY ROCKS. 47 



in A. rugosa (PL Y. fig. 2 a) ; but notlimg of the kind appears 

 in Goldfuss's larger figure of A. paradoxa (Taf. slix. fig. 9), the 

 margins of the basal grooves being quite simple and not plaited at 

 all. I imagine that this difference is merely one depending on the 

 state of preservation of the specimen ; for in the English examples 

 the plaiting is much more distinct at the sides of some grooves than 

 at the sides of others which are evidently more worn. 



The same is probably the case with regard to the differences 

 between the English and German specimens in the nature of the 

 openings on the upper surface of the centrodorsal. The central 

 axial opening in the latter, the so-called mouth, is distinct from 

 the five large oval holes disposed around it, the so-called " genital 

 openings." In the English specimen there is a five-pointed axial 

 opening, the points of which, rather less deep than the centre, seem 

 to correspond to the "genital openings" of Goldfuss's specimens. 

 This is probably due to accident, just as supposed by Geinitz * for 

 his specimens, the " hufeisenformig erhabene Leiste," which Gold- 

 fuss described as forming the central boundary of the pits, having 

 been broken in the removal of the matrix ; for the central ends of 

 some of the radial extensions of the axial opening have small pro- 

 jections from their sides, which look as if they were the remains of 

 broken bars bridging these extensions. I say " bridging," because 

 I do not think that these bars were the upper ends of very thin 

 walls separating the radial pits from the central cavity, as in 

 A. Metzii. From the condition of the specimen I should rather 

 judge that there was but one five-rayed cavity, and that the central 

 ends of the rays were bridged over by thin bony bars. The outer 

 ends of some of them have a tendency to bifurcation. This was 

 also visible in Goldfuss's specimens, as is well seen in his fig. 9 c on 

 Taf. xlix. It is a peculiarity of some interest, because, as already 

 mentioned, it is exhibited in a much more marked degree by one of 

 the ' Challenger ' Antedons. 



But the most curious feature in A, paradoxa is its possessing 

 certain characters which are confined, with onty one exception, so 

 far as I know, to two Antedons inhabiting respectively the northern 

 and southern extremities of the Atlantic. The one is the well- 

 known A. EscJiricJitii of the Arctic Ocean and the north temperate 

 zone, and the other is a closely allied but not identical form from 

 the Southern Sea (Heard Island). In both of these species the 

 centrodorsal cavity is rather deep, and its walls are marked by strong 

 ribs, the lower ends of which are more or less distinctly visible 

 through the axial opening, projecting beneath its lip which their 

 upper ends help to support. Eive of them, those at the inter- 

 radial angles, are often considerably larger than the rest, and may 

 be the only ones visible f. In other cases, however, both these and 

 numerous smaller intermediate ribs are visible through the axial 

 opening, as is seen in PI. Y. fig. 3. The ribs are much more 



* "Das Elbthalgebirge in Sachsen," Palaeontographica, vol. xv. pp. 91, 92, 

 Taf. xxiii. figs. 8-10. 

 t Trans. Linn. See. ser. 2, Zool. vol. ii. pi. iy. fig. 11. 



