File:The Quarterly journal of the Geological Society of London (14076440545).jpg

428 PKOE. p. M. DFNCA.!^ ON THE STETJCTTJEE OF
that the whole subject of the classification ought to he reinvestigated,
the morphology of the ambulacra being considered of primary im-
portance. It became easy, after the examination of weathered speci-
mens, to decide that whilst some recognized species oi Pseudocliadema
were evidently Oligopores and closely allied to the modern Diadema^
others were Polypores, having sometimes as many as fi.ve or six pairs
of pores to an ambulacral plate. Again, some species are allied to
the recent forms by having the optic pores at the actinal margin of
the radial plate, and by having decided branchial cuts and even tags
arising from the cuts. Moreover the structure of CypJiosoma being
known to me, I could hardly consent to so close an alliance as
Dr. Wright suggested between it and Pseudodiadema.
Pseudodiadema hemisj)hcericum is well drawn by Bone in Dr. Wright's
Monograph of the Erit. Foss. Echinodermata, pt. 1, 1855, plate viii.
The shape of the radial jDlates and the position of the optic pore at
the very margin of the plate are clearly indicated, and the drawing
of an ambulacrum (fig. . d) shows the relation of three pairs of pores
to each tubercle-bearing plate. The exact relation of the pairs is
not shown ; for the specimen was so perfect that no sutures probably
were visible. But in the British Museum there is a specimen
(No. 23329) from Malton, named, as of old, Diadema pseudodiadema,
and the lines of the sutures may be seen here and there.
In the great majority of the ambulacral plates there are three
pairs of pores. Each pair is in a primary plate, and the three pri-
maries have become fused, as it were, into a geometrical compound
plate (fig. 5).
Fig. 5 (see p. 451).
The first or aboral pair of pores of this compound plate has its
adoral pore in contact with the adoral suture of the low broad
primary plate which forms the first or apical portion of the com-
pound plate ; and this suture is directed to the median or vertical
suture in a course which is somewhat curved, the convexity being
adoral.
The suture crosses the boss of the tubercle just abactinally to the
mamelon.
The pair of pores which belongs to the middle plate of the
combination has its adoral pore in contact with a suture that unites
its adoral edge with the aboral edge of the third plate. The
direction of this suture is towards the median suture, and it has
a path from the interradial end of the poriferous zone to the