File:The Quarterly journal of the Geological Society of London (14073264751).jpg

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