File:The Quarterly journal of the Geological Society of London (14053342926).jpg

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.