File:The Quarterly journal of the Geological Society of London (13889849518).jpg

438 PEOF. p. M. Dtn!TCAJf OK THE STKUCTTJEE OE
From the effects of the ohliquity of the adoral edge, the part of
the plate near the interradium is greater in vertical measurement
than that towards the median line.
JSTumerous small and usually very low primary plates succeed, and
in the ambulacrum under examination, wherever a granule-bearing
plate is in contact adorally with a small tubercle-bearing one this
last intrudes upon and deforms the other (fig. 11).
Fig. 11 (see p. 452).
The plates 15, 16, 17, and 18, of the same ambulacrum and zone,
may be taken as typical of the greater part of the area halfway
between the great tubercles at the ambitus and the radial plate.
Plate 15 is a low primary with a very small mamiUated boss and
three minute granules. The interporiferous portion is on a slightly
higher level than the poriferous area, and the aboral edge of the
plate below delate 16) fits into a space which is produced by this
want of conformity of level.
Plate 16 is of the usual height in the poriferous area, but it is
forced up aboraUy by the expansion of the tubercle-bearing plate 17,
situated immediately actinally to it, so as to conform at its adoral
edge with the curve of the base of the tubercle. Hence this plate,
which is very sparely ornamented with a granule or two, is almost
linear towards the median line.
Plate 17 has a small tubercle which occupies nearly the whole of
a much expanded interporiferous area, the increase in growth being
apical. The next plate, 18, is a low primary with granules, and it
is not much deformed by the slight adoral extension of the above-
mentioned tubercle. All these plates are primaries.
Js'earer the ambitus the fusion of primaries, forming compound
plates with two pairs of pores, becomes evident. Thus at plates 35,
36, and 37 (fig. 12), this is weU seen.
Fig. 12 (see p. 452).
37
Plate 35 is a primary, or, rather, was one before its adoral edge