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430 PEOP. p. M. DTJIS'CIX ON THE STErCTTJEE OF
uncoalesced primaries. Near the peristome the simplicity of the
compound plate persists, and there is no crowding of the pores. An
Fig. 7 (see p. 452).
interesting structure exists in the form of a tag, which, as in the
recent Diademata, passes up from the branchial cut by the side of
the ambulacra. So far as the test is concerned, or rather that
part which remains, the apical system being deficient, there is
no distinction to be made between this species and a recent
Diadema.
There are many species included in the genus Pseudodiadema
by authors which have the simple triplet arrangement of pores
just noticed, and the peristomial crowding never amounts to a dis-
placement of pairs or the production of demi-plates. I have been
able to examine many of the forms described by Dr. Wright, thanks
to the courtesy of his executors. The type of this group existed
from the Inferior Oolite, if not from the Lias, to the Cretaceous
age inclusive ; and it is a matter of great interest to have been
told that hollow, striated, and verticillate spines were found in the
Chalk and drawn by Mr. Bone. The Pseudodiademata with simple
triplets form, therefore, one distinct type or group. The Pseudodia-
demata^ having also occasionally an additional pair of pores belonging
to a demi-plate, belong to a closely allied section or subgenus or group.
The next tj'pe to be considered, the third group, is one in which
there are never less than four plates and four pairs of pores to a
compound plate, and of which Pseudodiadema mamillanum already
alluded to, is an example.
In most of the specimens of this series there is barely any trace
of the divisional sutures to be seen in those plates which have the
greatest number of pores ; but an instance of a form clearly pre-
senting all the necessary structures to view from which a drawing
can be made is in Dr. Wright's collection*. Having examined the
specimen, it is evident that a compound plate at the ambitus has no
less than five primary plates entering into its composition, and that
the next above or aboral compound plate has four (fig. 8). The pairs
of pores are in slight arcs, the peripodia are well developed and often
occupy nearly or quite the whole height of the poriferous area of
the plate on which they are placed. The adoral pore of a pair is
always in relation with the suture between its plate and that

• Pseudodiadema Michelini, Agass.