File:The Quarterly journal of the Geological Society of London (14073284602).jpg

THE AMBULACRA OF FOSSIL ECHINOIDEA. 4:25
spending plate of a modern Diadema. The middle plate is lowest at
the centre of the tubercular area, for it is nipped in by the curved edge
of the first plate and also bj^ an aboral curvature of the third plate.
But further towards the median line the middle plate, after carrying
the mamelon, expands, and is in relation with much of the boss
and the greater part of the compound plate. The third i^late has
its aboral edge curved apically, and it is a low primary, for the
suture between it and the middle plate reaches the median suture
just abactinally to the adoral and inner angle of the compound plate.
The resemblance of these plates to those of the typical and recent
Diademata is exact.
Perhaps the most striking species of Hemipedina is H. tuhercidosa,
"Wright (op. ciL p. 164), on account of its resemblance to a Hemi-
cidaris without crenulation, and with an unusual number of small
secondary tubercles in the interradia. It is a beautiful form, and is
even more Cida^^is-looking than Hemicidaris. The ambulacral
plates, however, do not always remain as simple primaries; for
towards the ambitus, where the tubercles increase in size rather
suddenly, there are three pairs of pores evidently in relation to as
many plates which have combined to produce a geometrically shaped
compound plate (fig. 4). The triple pairs may arch very slightb
Fig. 4 (see p. 451).
around or be straight at the edge of a great tubercle, which nearly
covers the entire plate. The peripodia, which are only slightly
oblique and broadly elliptical in shape, are not so crowded as they
are in Hemicidaris ; but they impinge upon the outer flank of the
tubercle, and in some specimens their relation to the plates which
their pores perforate can be appreciated.
Taking the first tubercle above the ambitus, it will be noticed to
be situated apically to a decidedly large one, and to be separated
from it by more space than exists between the other tubercles placed
in succession towards the peristome. The three peripodia are in a
slight arc, and the most adoral is slightly nearer the tubercle than
the others. The tubercle is a broad, low cone, with a well-developed
mamelon surrounded by a decided groove. Careful observation
proves that the adoral pair of pores has the adoral pore on a line
with a transverse suture which separates the combination to which
this poriferous plate belongs from the next plate in actinal suc-
cession. And on the adoral flank of the tubercle, and nearer the
base than the groove around the mamelon, is a line which can be
traced from the adoral pore o£ the peripodium which is the middle
one of the triplet, over the slope of the boss to the median line of