File:The Quarterly journal of the Geological Society of London (13889859850).jpg

THE AMBULACRA OF FOSSIL ECmNOIDEA. 439
this union produced a symmetrical plate towards the median line.
The tubercle of plate 36 intrudes on plate 35, the adoral suture of
which crosses the boss to reach the median line not far from the
apical angle of the compound plate : this suture is curved, with the
convexity directed adorally. A few granules are placed between
the tubercle and the median line, and upon the apical part of
plate 35.
On comparing plates 35 and 36 with plates 16 and 17, it would
appear that the resisting power of the plate 35 was greater than
that of plate 16; but this is the first evidence we have of the
direction of the aboral plate of a combination. It is important to
observe that the poriferous area of plate 35 is higher in measurement
than the part close to the median line.
The tuberculiferous area of plate 36 is much larger than the
corresponding poriferous area. The next plate, 37, is a small low
primary, with granules, and the first step towards its organic
connexion with plate 36 is the symmetrical arrangement of the
granules in conformity with their place and trend in the two other
plates 35 and 36, which certainly do form a double-pored combi-
nation.
The structure of the plates immediately aborally to the first large
tubercle at the ambitus, counting from the radial plate, is simple as
a rule, and differs in a remarkable manner from that of the great
tubercle-bearing compound plates. Taking the Amb. III. of the
species of Remicidaris, and the zone " a," it will be noticed that the
small compound plate placed aborally to the great tubercle, consists
of three low primary plates united to form a symmetrical com-
bination (fig. 13)*. The tubercle which is placed on this compound
Pig. 13 (see p. 452).
plate is small, and its boss is crossed, from the poriferous area to the
median line, by the sutures of the aboral and adoral plates (37 & 39)
where they are in contact with the edges of the central plate, which
usually has the mamelon upon it (38). The line of the sutures is almost
transverse in most instances ; but it may happen that the adoral
plate of the combination is so jammed by the huge tubercle of
the plate immediately on the actinal side, that it becomes a demi-
plate, because it cannot reach the median line (fig. 14).
The following is the construction of a large tubercle-bearing plate
at the ambitus (figs. 13, 14).

• Specimen 24122, British Museum.