File:The Quarterly journal of the Geological Society of London (14073246131).jpg

424 PEOF. p. M. DUNCAN ON THE STPvTJCTTJEE OF
Geological Survey, Jermyn Street, and in most the number of single
primary plates near the apical end is very striking, indeed almost
leading to the idea that all the parts of the ambulacrum are made up
of them. But there are one or two compound plates preserved. In
one of these, near the peristome, the triple nature is evident, and the
line of suture between the middle and adoral plates is curved, and
passes from the adoral pore of the middle pair towards the mamelon
of the tubercle and thence to the median or vertical suture, the con-
vexity of the curve being directed apically. In another specimen the
line of suture between the middle and the aboral plate is seen, and
it is curved, with the convexity towards the mamelon. Hence these
compound plates are on the Diadematoid type, and it may be presumed
from the phenomena presented by the recent species of Diadema that
growth-pressure has changed the shape of the original primaries.
In one of the specimens in the Museum there are four plates and four
pairs of pores in one of the compound plates, and it appears, but
not very satisfactorily, that the additional plate is a low primary.
This is not without its significance ; for a similar structure is seen
in allied genera *.
Hemipedina marehamensis, Wright, from the Coral Hag, is a fine
form belonging to a section of the genus which has numerous
primary tubercles in rows on the interradia.
There is a specimen in the British Museum (no. 75923) which
shows the shape of the triplet of plates which combine to form a
geometrical plate near the ambitus. The compound plate is broader
than high, and there is a space between the tubercle and the median
or vertical suture (fig. 3). The direction of the sutures between the
Pig. 3 (see p. 451).
three plates indicates their shape, especially as the whole compound
plate is contained between an aboral and an adoral transverse
suture.
The pores are rather oblique, and the adoral pore of the first pair
is on the suture which unites the first and middle plates. The
line of this suture is, from the interradium to the adoral pore, and
thence with a curve, convex adorally, up the flank and over the top
of the boss of the tubercle apically to the groove at the base of the
mamelon, and then down the slope to the edge of the boss to reach
the median suture at a short distance from the aboral and inner
angle of the compound plate.
This first plate is therefore a low primary resembling the corre-

• With regard to Hemipedina BowerhanTcii, Wright (op. cit. p. 145), illus-
  

trated on plate ix. of the Monograph already noticed, it must be observed that
the figure 2 b cannot be correct. It represents the pores as if they were turned
upside down, and the adoral pore, or that which is furthest from the inter-