NEW FEATUEES IN PEL ANE CHINES COEALLINTJS. 



707 



plates are seen very distinctly. The primaries can readily be traced 

 across by means of the granulations and tubercles covering them. 

 The sutures traverse the base of the large tubercle, which thus 

 rises from all three primaries. 



I have described the structure of these plates at length because 

 I believe the type to be a new one. It does not seem to be included 

 under any of the six types given by Prof. Duncan, in his paper on 

 the ambulacra of fossil Echinoidea * Of these six the only ones 

 it at all resembles are the Echinoid and Diadematoid ; from the 

 former it differs in having primaries in the middle, and from the 

 latter in having demiplates in the middle. 



This structure may, however, have arisen from the fusion of three 

 oligoporous plates into one ; thus each of the primaries with the two 

 demiplates above would represent an oligoporous plate. Three of 

 these fusing with one another, together with development of the 

 middle one with its tubercle at the expense of the two others, would 

 give us such a polyporons plate as we have before us. 



This phenomenon seems to me quite analogous to the method of 

 formation of the oligoporous plates from three primaries, especially 

 in the case of many Diadematoid and Arbacioid forms in which the 

 middle plate develops at the expense of the other two. Whilst enter- 

 taining this idea, 1 was much interested to find that towards the 

 apex the association between the primaries seems much less inti- 

 mate, the sutures being replaced by deep fissures, and the adoral 

 and aboral primaries with their tubercles relatively larger and more 

 independent ; in fact I believe that in the uppermost plates seen, where 

 the most primitive condition doubtless prevails, the oligoporous 

 condition actually obtains, though the plates are not quite of the same 

 size. 



Traced towards the mouth the structure of the ambulacral areas 

 is precisely the same as at the equator, and the arrangement is still 

 trigeminal. At the peristome the imbricating plates come in sud- 

 denly. As the area narrows, the tubercle decreases in size, and the 

 middle primary ceases to project much beyond the others, so that 

 the median ambulacral line running at the equator in sharp S-like 

 curves passes, through less abrupt curves, into a gently undulating 

 line finally terminating in an almost straight portion. 



Above the equator the main tubercle again diminishes in size, but 

 much more suddenly, in correspondence with a more sudden narrowing 

 of the areas. As the middle primary diminishes in size, the adoral 

 and aboral primaries maintain theirs, and their tubercles increase 

 rather than diminish in size, so that at the highest point seen the 

 primaries with their tubercles show comparatively little difference 

 (fig. 3). The plates here seem to be in an oligoporous condition, 

 and judging from the diminished width of the areas, the small and 

 nearly uniform size of the tubercles, and the gently undulating 

 median line, the ambulacral areas are near their aboral termination. 



The ambulacral plates, then, of PeJanechinus may be looked upon as 



* Quart. Journ. Geol. Soc. vol. xli. 188 



