THE AMBULACRA OF FOSSIL ECHINOIDEA. 441 



the flank of the boss adorally to the crenulated ridge, and which then 

 turns with a more or less wide angle to reach the suture between 

 the tubercle-bearing plate now under consideration and that placed 

 immediately adorally. But the suture does not reach the median 

 line of the ambulacrum, for the plate (42) which it bounds aborally 

 is a demi-plate *. 



This demi-plate, the third or adoral, of the compound tubercle- 

 bearing plate (no. 42), varies in size in different tubercles and is 

 always highest at the region of the boss. 



The central plate (no. 41), which is bounded actinally by the 

 suture first described which crosses the tubercle obliquely, and 

 also by that just noticed, is very large and is a primary. It is rather 

 low at the poriferous area, but the height increases o"n the surface 

 occupied by the mamelon, and all the inner part of the combination- 

 plate is occupied by it towards the median line, except a small por- 

 tion close to the aboral and inner angle which belongs to the highest 

 of the triplet. Thus the great tubercle-bearing compound plate is 

 composed of a large intermediate primary, a smaller aboral primary, 

 and a large demi-plate (sometimes a primary) which is the adoral 

 of the three. 



It is interesting to note that in shape the aboral plate re- 

 sembles the corresponding plate in the genus Diadema, and that 

 the adoral has frequently the outlines of the corresponding plate 

 in Coelopleurus, Duncan and Sladen, op. cit. 



All the great tubercle-bearing plates of Hemicidaris show the 

 details just described, and if there is any variation it depends on 

 the position and nature of the plate towards the peristome where 

 the sutures come closer together. 



In the majority of ambulacra the next three plates, situated 

 adorally to those just described, carry a large tubercle resembling 

 in shape that noticed above, but smaller. The plates form a com- 

 pound one, and the lines of their sutures and therefore their shapes 

 are the same as in the compound plate just noticed. The peri- 

 podia, are in an arc, and are three in number, and the aboral plate 

 of the triplet is a broad and low primary resembling that of the 

 first tubercle-bearing plate ; the second is also a large primary, and 

 the third or adoral plate is a demi-plate. 



The next compound plate is also a triplet, and so are all the others 

 down to the peristome. The pairs of pores and their peripodia are 

 closer, and the arcs are interfered with in consequence of growth and 

 pressure; but the three peripodia of a tuberculiferous plate can 

 always be distinguished, although the second peripodium of a series 

 may be almost excluded. There is no addition of plates or pairs of 

 pores, and the position of the pairs corresponds with that observed 

 and described by Loven in Stronrjylocentrotus, although the expla- 

 nation he gave will hardly meet the instance of Hemicidaris. 



An important exception to the rule regarding the regular sequence 

 of the ambulacral tubercles, occurs in some specimens. Thus in a 



* There is some variation in different specimens and the suture does reach 

 the median line in some (see fig. 13). 



