50 FOSSIL ASTEROIDEA. 



The position of the madreporite is nearer the centre of the disk than the 

 margin. 



The infero-marginal plates are six in number, counting from the median inter- 

 radial line to the extremity, — that is to say, there are twelve for the whole side of 

 the disk, as against six in the supero-raarginal series. The length of the two 

 innermost plates on each side of the median interradial line is subequal to that of 

 the superior series, but the four succeeding infero-marginal plates are much shorter, 

 and together correspond to the large ultimate supero-marginal plate. As seen in the 

 lateral wall of the disk the height of the infero-marginal plates is very little more 

 than one-half that of the superior series, the actual proportion in the specimen 

 under notice being less than three-fifths. The breadth of the plates adjacent to 

 the median interradial line on the actinal surface is 7*5 mm. in an example whose 

 major radius measures 25 mm. and the minor radius 20 mm. The breadth of the 

 marginal border diminishes so rapidly on each side of the median interradial line 

 towards the extremities of the rays that the inner margin of the series of infero- 

 marginal plates of each side of the disk is practically a sector of a circle. The 

 surface of the plates is marked with small widely spaced punctations, and there is 

 a narrow depressed border round the entire margin of the plate which is very finely 

 punctate ; the border is broader at the adcentral end of the plate and the adjacent 

 corners than elsewhere. 



The adambulacral plates are small, broader than long, and their surface is 

 marked with about three oblique series, and an outer irregular group of fine 

 punctations upon which the spinelets composing the armature of the plates were 

 articulated. There are about six punctations in the innermost ridge or series 

 adjacent to the ambulacral furrow. A few spinelets are still preserved in an 

 example from the Upper Chalk near Bromley, from which this description is taken, 

 which is in the British Museum collection, and bears the registration number, 

 " B 2613." The spinelets are cylindrical, truncated, and do not appear to taper ; 

 and their length is about equal to the length of the plates. 



The actinal intermediate plates are rhomboid and hexagonal or polygonal in 

 form, and their surface is entirely covered with small equidistant punctations, 

 upon which small uniform granules were previously attached. Remains of this 

 granulation are occasionally to be found preserved in siUi, and may be seen in the 

 example referred to in the preceding paragraph. Small entrenched pedicellarisB 

 are occasionally present on the actinal intermediate plates. 



Dimensions. — The example figured on PL XTV, fig. 2 a, has a major radius of 

 36-5 mm. and a minor radius of 27 mm. The length of the side is about 40 mm. 

 Another specimen, shown in fig. 1 a on the same plate, is rather larger, the length 

 of the side being about 45 mm. Much displacement of the plates has occurred in 



