THE AMBULACRA OF FOSSIL ECHPNOIDEA. 449 



and are then lost at the very base of the large imperforate mamelon. 

 The lines converge, and as the other lines of union of the plates 

 nos. 1 & 4 already mentioned also converge, a very marked feature 

 results : but there are no lines in continuation of those numbered 

 2 & 3 to be seen on the part of the tubercle near the median line 

 and between the two very distinct lines already noticed. Conse- 

 quently there must be at least two demi-plates, also the aboral and 

 adoral primary plates, besides a middle primary. 



There is sometimes an appearance in compound plates consisting 

 of five plates, as if the third plate from the abactinal edge were the 

 middle primary (fig. 23). The fourth plate appears to be a demi- 

 plate which joins on to the fifth or the adoral primary. 



This arrangement would be very exceptional amongst forms of 

 Echinoidea with many demi-j)]ates ; nevertheless the position of the 

 middle primary would be that seen in the Diadematidse of the 

 recent fauna, with the addition of a demi-plate placed aborally and 

 adorally to the middle plate. 



It is also to be remembered that Alex. Agassiz gives a diagram of 

 Pliymosoma (Ci/phosoma) cremdare, Agass., in the ' Revision of the 

 Echini,' plate vi. fig. 2, in which the middle primary has a demi-plate 

 on either side of it, and the arrangement is the same as that now 

 under consideration. But the species mentioned by Agassiz is not a 

 Ci/jjJiosoma, for the abactinal pores are not diplopodous ; the form 

 must come within a new genus. 



This diplopodous condition of the pores must be considered in 

 investigating the formation of the ambulacral compound plates. 



In a specimen of Cyj^liosoma Kojiigi, Mant., there is a compound 

 plate higher than the ambitus, in which the diplopodous condition 

 lingers on, and there are three double sets of pores (fig. 24). 



Fig. 24 (see p. 452). 



The rule is followed in the compound plate, with regard to the 

 succession of the pairs, which prevails more abactinally and where 

 the double sets are not united in compound plates. The outer pairs 

 of pores are in plates which are crushed and crowded out from the 



