File:The Quarterly journal of the Geological Society of London (14076437265).jpg

THE AMBULACKA OF POSSIL ECHINOIDEA. 423
broad primaries near the radial plate, and extending far towards
the ambitus, and each plate is united by a separable suture to
the plates situated apically and actinally. At the ambitus three
primaries are seen to form a compound plate geometrical in figure,
and their separability is at an end. There are triple plates of
this character down to the peristome. Taking a compound plate
at the ambitus (fig. 2) as the example, it appears that it is
Pig. 2 (see p. 451).
broader than high, and that the pairs of pores, three in number,
are either straight or in a very slight curve. Two lines of suture
cross the compound plate, and they indicate the edges of the
primaries which have united to form it. One line is between the
aborally situate plate and the middle one of the combination, and
it passes from the adoral pore of the aboral pair inwards to
cross the base of the tubercle apically to the mamelon, and thence
it passes on to the median suture of the compound plate or,
as it is called, the vertical suture or edge. This line is very
faintly curved with the convexity towards the tubercle, or it
may be straight and cross the fiank of the boss. The other sutural
line is between the middle plate and the adoral plate, and it is in
contact with the adoral pore of the second or middle pair ; thence
it crosses the adoral flank of the boss actinally to the mamelon
and reaches the vertical suture with or without a curve. The
aboral plate of the combination is a low and broad primary which
includes a small portion of the boss, and the adoral plate is
of the same configuration as the other. The middle plate is rather
the largest, and is a well-formed primary which carries much of the
boss of the tubercle and all the mamelon.
The sutural lines between the. plates constituting the compound
plates are in some instances decidedly curved, with the convexity
placed towards the mamelon ; but this condition is replaced nearer
the peristome by quite straight lines. The formation of the almost
rectangular primaries in the combination resembles a simple appo-
sition of Cidaris-like plates, and it is interesting to notice this
primitive type merging gradually into the fully developed one, in
which, as in the ancient and modern Diadematoids, the curving of the
sutural lines is coincident with an enlargement of the middle plate
towards the median line at the expense of the plates above and
below, these plates then departing from the shape of those of the
Cidaridse.
Hemipedina Boweebankii, Wright.
There are some fine examples of this species in the Museum of the