THE AT\rBITLA.CRA OF FOSSIL ECHIN"OIDEA. 443 



Fig. 17 (see p. 452). 



It appears, then, that in the genus Hemicidaris the multitude of 

 small primaries is succeeded by doublets or triplets and the great 

 tubercle-bearing plates. These are either triplets after the Diadema- 

 type, more or less modified, or have four pairs of pores, the additional 

 pair being in a low primary, which has been joined to the aboral 

 edge of a compound plate, or in a demi-plate (fig. 15). The influence 

 of the growth of the large tubercles upon the spreading of the middle 

 plate and the curvature of the adoral and aboral plates is very 

 evident. 



Pinally, it appears that the arrangement of the triplets when 

 crowded at the peristome is not very remote from that seen in some 

 abactinal parts of the ambulacra of species of Pedina. 



Genus Diplopodia, McCoy. 



Small immature specimens of such types as Pseudodiadema versi- 

 pora show a doubling of the pairs of pores near the apex unlike the 

 condition which prevails in small specimens of the true Pseudo- 

 diadema, which are unigeminal. This species, according to the 

 principles which govern the classification of the recent Echinoidea, 

 cannot remain in the genus Pseudodiadema, and must come within 

 Diplopodia. 



Other forms are said to become diplopodous only at adult age, and 

 this has been considered a sufficient reason for not placing them out of 

 the genus Pseudodiadema ; but it was forgotten by the adopters of 

 this reasoning that zoologists must consider the adult development 

 of a form, and not its immature condition. A form with bigeminal 

 pairs of pores in the upper part of the ambulacra is a Diplopodia ; 

 and as yet I must confess not to have been able to recognize any 

 forms about to become diplopodous. We can only deal with absolute 

 facts, and not with presumptions. 



The question arises, leaving out the bigeminal nature of the pairs, 

 Are the other generic characters sufficient to separate Diplopodia 

 from Pseudodiadema ? 



It appears that the diplopodous condition near the apex is accom- 

 ■panied.by crowding and doubling of the pores near the peristome, 

 and, as a rule, by some departure of the pairs of pores in plates at 

 the ambitus from a regular line or arc. 



Moreover, the structure of the part of the ambulacra near the 

 radial plates differs in the di]3lopodous series from that seen in the 

 true Pseudodiadema. There is not that blending of the small plates 

 into compound ones which is a character of Pseudodiadema. 



When there is such a combination as in Diplopodia Roissyi, Cott. 



