ON SOME NEW FEATTJEES IN PELANECHTNTTS COKALLINUS. 703 



49. On some New Feattjkes in Pelanechintts corallines. By 

 T. T. Gteoom, Esq., of St. John's College, Cambridge. (Read 

 June 23, 1887.). 



[Communicated by Prof. T. M C K. Hughes, F.G.S.] 

 [Plate XXVIII.] 



In vol. xxxiv. p. 924, of the ' Quarterly Journal of the Geological 

 Society,' Mr. Walter Keeping, F.G.S., described two specimens of a form 

 previously referred by Dr. Wright to the genus Hemipedina. He 

 showed that this form was quite distinct from Hemipedina, and gave 

 it the name of Pelanechinus. It showed, in his opinion, affinities 

 with the Echinothuridse on the one hand and the Diadematidae on 

 the other. 



These two specimens have hitherto been the only ones known, 

 with the exception of a few fragments described by Dr. Wright. 



I was fortunate enough to find in 1881 in the Coral Rag at Calne, 

 in a quarry at the north end of the town, a third specimen which 

 shows some interesting features hitherto undescribed. 



The diameter of the test is about 85 millim. ; it is considerably 

 flattened from above downwards. The whole of the oral surface is 

 beautifully preserved, with the exception of a sector including one 

 ambulacral and the greater part of two interambulacral areas. The 

 teeth are exposed in situ, together with most of the peristomial 

 imbricating plates ; the latter, however, are not exhibited so clearly 

 as in the specimen in the Woodwardian Museum. A zone, about 

 15 millim. wide, belonging to the oral surface is also preserved. My 

 specimen quite confirms Mr. Keeping's opinion that the test was 

 flexible. 



The plates have frequently separated along the line of suture, 

 especially on the aboral surface, while the median interambulacral 

 line and the lines of junction between the ambulacral and interam- 

 bulacral areas have frequently broadened into gaping fissures ; the 

 calcareous matter between the plates must either have been deficient 

 or wanting ; the plates themselves are occasionally bent, but very 

 seldom broken. 



The Interambulacral Areas (fig. 1) differ considerably on the under 

 and upper sides of the test. At the equator there are two large 

 median tubercles to each plate, and two smaller lateral ones ; 

 towards the mouth all these decrease regularly in size, and at a 

 little distance from the peristome diminish first to three, and then 

 to two on a plate, the two innermost rows (i. e. nearest the median 

 interambulacral line) disappearing first, while the outermost of the 

 large tubercles are the only ones that are represented on the last 

 plates. Above the equator a similar diminution in size takes place 

 in three of the rows ; the innermost row of large tubercles almost 

 immediately disappears, as also does the outermost row of the 

 smaller tubercles. The outer row of large tubercles retain almost 

 their full size. 



