some Species ri furred to it. 59 



the individual plates may occasionally be convexly curved, 

 and a slight accentuatiou of this appearance might give rise 

 to the idea that tlie plates were subhexagonal or heptagonal. 

 But that these are the adambulacral margins, and that the 

 interamljulacrals therefore are pentagonal, is conclusively 

 proved by one portion of an interambulacrum. This con- 

 sists of two columns, with apparently six plates to each 

 column, and its iuiier surface is fully exposed (PI. I. fig. 9). 

 This in all essential details resembles the similar inner view 

 of an interambulacrum of C. heyserUngi from Possneck ; 

 it has the same denticulate margins and the same thickenings 

 at the peristomial end for the jaw-muscles. It confirms not 

 only the view that the two specimens belong to the same 

 species, but also the reference of that species to Miocidaris. 



The secondary tuberculation of the interambulacrals from 

 Tunstall Hill displays slight variation. In some the whole 

 extra-scrobicular surface is filled with closely set secondaries 

 of equal size (PI. I. fig. 12) ; in others the tubercles of 

 the scrobicular ring are slightly, but distinctly, larger than 

 those in the interradial space (PI. I. fig. 11) ; in others 

 again the tubercles are less closely set, and bare tracts are 

 seen between them (PI. I. figs. 10, 13). The development of 

 the secondary tubercles is of course greatest in the ambital 

 region and in individuals of largest size ; but apart from this 

 there are individual differences. The relative width of the 

 interradial tract also varies, aud the wider tracts naturally 

 have more tubercles j thus in an interambulacrum 8 mm. 

 wide at the ambitus, the width of the interradial tract, in- 

 cluding the scrobicular rings, is 2T mm., and there are 5 

 or 6 tubercles in that width (PL I. fig. 13) ; in another 

 specimen the corresponding measurements are 9'4 mm. and 

 3*5 ram , and the number of tubercles is 7 or 8 (PI. I. fig. 12). 



The secondary tubercles, when well preserved, are seen to 

 have small, apparently imperforate mamelons. 



The material from Tunstall Hill includes 7 radioles ; 

 the one figured by King came from Humbleton Hill and was 

 the property of Mr. Geo. Tate. Of these 7, the longest 

 complete (or almost complete) one is just 8 mm. long, and 

 the greatest diameter of its shaft is I'l mm., this being at 

 about one-third from the distal end. The greatest thickness 

 exhibited by any of these radioles is V^ mm., but this is 

 largely due to the prominence of the thorns. The annulus 

 is prominent, with a diameter nearly equal to that of the 

 shaft — 1 mm. in the first-mentioned example ; from it a 

 straight slope leads to the crenelate acetabular rim. No 

 definite collerette can be distinguished, but the proximal 



