THE BRITISH SECONDARY ROCKS. 49 



2. Antedon rugosa, n. sp. Plate Y. fig. 2. 



The centrodorsal is a thick peutagonal disk with rounded angles 

 and steep sides, which bear about twenty-five cirrhus-sockets in two 

 irregular alternating rows. The sockets have keyhole-shaped per- 

 forations and striated margins like those of A, iniradoxa. The 

 dorsal surface is flattened, with a deep central hollow, at the bottom 

 of which is a five-rayed impression. In the centre of the ventral 

 surface is a shallow five-rayed cavity without any ribs on its sides. 

 The rays have straight smooth sides, without any indications of plates 

 or bars separating them from the central space ; so that I do not 

 imagine the stellate shape of the cavity to be accidental, as Schluter * 

 suggests in the case of A. semiglohos.a. The basal grooves have 

 plaited sides, but are less deep than in A. paradoxa ; in fact one 

 of them slopes gradually downwards at its central end on to the 

 floor of the central cavity, and is not raised slightly above it, as is 

 the case with the other four. The angles of the ventral surface, 

 where their outer ends lie, project rather prominently (PI. Y. 

 fig. 2, c). Beyond the ends of the rays of the central cavity there 

 is a furrow in the middle line of each radial area ; its depth is 

 rather variable, for it is sometimes distinct and sometimes barely 

 traceable, e.g. the two furrows at the sides of that basal groove 

 which is so much lower than the rest. 



Diameter 9-5 millims. ; height 3'5 millims. 



Loccdity. Chalk (Sussex). Dixon Collection, British Museum. 



Remarks. This species, while resembling A. iDaradoxa in the 

 characters of its cirrhus-sockets and basal grooves, presents many 

 points of difference from that type. It is flatter, with a more pen- 

 tagonal outline and fewer cirrhus-sockets ; while there are no ribs 

 on the walls of the central cavity, the appearance of which is very 

 different from that of Goldfuss's species. The striated cirrhus- 

 sockets, pentagonal outline, and stellate axial cavity (?) are points of 

 resemblance to A. semiglohosa ; but the latter species has a hemi- 

 spherical centrodorsal with nearly fifty cirrhus-sockets, and the 

 sides of its basal grooves are not plaited. 



3. Antedon ^quimarginata, n. sp. Plate Y. fig. 4. 



This is a well-preserved specimen, consisting of the united radial 

 pentagon and centrodorsal piece. The latter is a truncated hemi- 

 sphere (9 "5 by 3-5 millims.), with a wide but shallow depression in 

 the middle of its under surface, and three incomplete rows of plain 

 cirrhus-sockets, placed more or less vertically above one another, on 

 its sides, seventeen or eighteen sockets in each row. Its ventral 

 surface is a good deal wider than the base of the radial pentagon, 

 the angles of which approach its margin (fig. 4, a) ; but there is a 

 considerable space between the incurved lower border of each radial 

 and the corresponding side of the roughly pentagonal centrodorsal. 

 These spaces gave support to the proximal portions of the second 

 radials, which would thus be partially invisible, the first, of course, 



* Op. cit. p. 42. 

 Q. J. G. S. No. 141. E 



