THE AMBULACRA OF FOSSIL ECHINOIDEA. 437 



all others, according to its founder and Desor, by the structure of 

 the ambulacra of the species. Desor remarks (' Synopsis dcs 

 Echinides Fossiles,' p. 51, plates x. and xi.) : " The distinctive 

 character is found in the ambulacra, which, in one part of their 

 length, particularly towards the base and sometimes as far as the 

 ambitus, are furnished with true tubercles, which are smaller than 

 those of the interradia, but which like them are distinctly crenulated 

 and perforated." The generic diagnosis also notes that the pori- 

 ferous zones are composed of two simple rows of pores which are fre- 

 quently doubled at the peristome. 



If the narrowness of the ambulacra and the multiplicity of the 

 pairs of pores are added to the above very definite characters, all 

 that has hitherto been recorded about the morphology of the 

 ambulacra will be found to have been stated. 



But the exceeding narrowness of the ambulacra above the ambitus, 

 and the relatively narrow interporiferous area, coupled with the 

 curving of long series of pairs of pores in relation to the great 

 interradial tubercles placed near to the ambulacra, are necessary 

 additions to the diagnosis. The species are numerous, and are 

 Oolitic. Many of the forms from the Coral Eag are well preserved, 

 and show structures which have hitherto escaped notice and which 

 are of considerable classificatory importance. 



In Hemicidaris intermedia, Porbes, and Hemicidaris crenularis, 

 the plates of the ambulacra near the radial end are simple primaries. 

 Each pair of pores is in a separate plate which either is ornamented 

 with a tubercle or only carries one or more granules (fig. 10). 

 There is usually an alternation of small tubercle-bearing and 

 granular plates. The sixth plate from the radial plate, on ambu- 

 lacrum II. zone a (fig. 10), specimen in Brit. Mus. no. 14122, may 

 be taken as a type of the small primaries at this part of the 



Eig. 10 (see p. 452). 



ambulacrum, where the plates are numerous and narrow. The 

 position of the peripodium is usually slightly oblique and close to the 

 interradial edge, near the adoral suture. The plate is broader than 

 high, and much of it is covered by the tubercle. This is low and 

 has a mamelon which is perforated. The granule-bearing plates of 

 this part resemble that just described, except that a granule occupies 

 the position of the tubercle. Usually there is but one granule. 

 The tubercle -bearing plate intrudes upon the granule-bearing plate, 

 which is placed aborally, so that this last is often the smaller of the 

 two. 



Besides the usual extension of the tubercle of the small primary 

 plates very high up in the ambulacrum, an oblique direction of their 

 adoral edge is often noticed ; and it is evidently the result of the 

 upward expansion of a plate which is situated adoraUy. 



Q. J.G.S. No. 163. 2h 



