File:The Quarterly journal of the Geological Society of London (14076926384).jpg

440 PROP. p. M, DITN-CAN 02f THE STETJCTXTEE OF
Fig. 14 (see p. 452).
The tubercle covers nearly the whole of the three plates which
compose the compound plate, and even the peripodia are on its
slope.
The tubercle, as is well known, is large and tall, has a sloping
boss, a wide crenulated ridge and groove, and a large perforated
mamelon. There are three pairs of pores surrounded bj' as many
peripodia in immediate relation with the plate, and they are rather
distant and in an arc. (A pair situated adorally to the others is in
connexion with the compound plate placed immediately actinally.
Again, a pair which is on a line with the aboral edge of the tubercle
belongs to the plate above.) On examining most specimens the only
trace of a suture between any of the component plates is seen very
generally as a depressed line on the side of the boss towards the
median line of the ambulacrum and passing towards the aboral and
inner angle of the compound plate or rather of the tubercle. The
direction of the hue is apical and to the median line, and it
reaches this last either slightly or considerably below the aboral
angle of the compound plate at the vertical suture. But in many
weathered specimens there is another and distinct suture visible,
and it passes actinally from the crenulated edge over the adoral face
of the boss, and it may reach the transverse suture with a gentle
curve. On the poriferous side of the tubercle the first-mentioned
suture is seen to be in relation with the highest of the three
peripodia of the plate, to commence in the line of groove passing
adorally to the obliquely placed first pair of pores, no. 40, and to
reach up the side of the boss to the crenulated ridge, and then to
cross the boss towards the median line. The direction of this suture
may be in a right line or in a slight curve with the convexity
looking actinally. The suture joins the aboral and central plates of
the triplet, 'and it leaves the mamelon adorally and pursues a more
or less oblique course. It is clearly touched by the adoral pore of
the pair.
The shape of the suture and its direction determine to a great
extent the shape of the aboral plate of the combination, and this
is a primary plate with the poriferous area higher in vertical
measurement than the opposite extremity, and with the inter-
mediate part the highest of all.
The next pair of pores, situated adorally to the last, are also
obliquely placed, and on the edge of the boss (no. 41), and the
adoral pore is in contact with a suture which passes inwards and up
I