830 DR. G. J. HINDE ON RECEPTACULITID®. 
head of the spicules, which in all the Receptaculitide is either 
rhomboidal or hexagonal in figure, very thin at its margins, but 
thicker at the central junction of the spicule, where it unites with 
the other rays. By those who have written on the group, this 
spicular plate has been regarded as a distinct integral portion of the 
organism, independent of the rays beneath, whereas it appears to 
me to have formed a constituent part of the spicule with which it 
was connected at the central point of junction with the horizontal 
and vertical rays. If the plate were separate from the rest of the 
spicule and formed by itself a dermal plate or surface-covering over 
the organism, it is difficult to explain how it would have been 
retained in its position ; in the instances in which it is absent, this 
arises from weathering or the effects of fossilization, which have 
similarly affected the rays beneath, 
In no other hexactinellid sponge, so far as I am aware, are there 
any spicules with similarly constituted head-plates; in many, how- 
ever, no sixth or summit-ray is developed; but in some of the 
abnormal spicules of the Carboniferous sponge, Hyalostelia Smithu*, 
Young and Young, sp., the sixth ray is in the form of a rounded 
knob. We have only to consider that the sixth ray in the spicules 
of the Receptaculitide, instead of being contracted to a knob merely, 
as in the Carboniferous sponge, has been developed in a horizontal 
direction, and by additions to its margins has assumed the regular 
rhomboidal or hexagonal figure by which it is adapted to fit in with 
the adjoining spicular plates to form an exterior layer to the or- 
ganism. Strong confirmatory evidence of the theory that the 
summit-plates of the spicules are modifications of the sixth ray in 
the ordinary hexactinellid spicule, is afforded by the small blunted 
knob which projects in the centre of these summit-plates in the best- 
preserved examples of Spherospongia, and traces of which are also 
present in Acanthochonia. In these forms we find the commence- 
ment of the sixth or summit-ray in the small central knob, from 
which, as a centre, the plate is developed horizontally by successive 
marginal additions. 
But though, in regular hexactinellid sponges, spicules with hori- 
zontal summit-plates are unknown, yet in certain lithistid sponges 
there are spicules with specially modified summit-plates, adapted 
for the outer surface of the sponge, which may; though in a some- 
what distant degree, be compared with the spicular plates in the 
Receptaculitide. Thus, in the Cretaceous genera Plinthosella, Zitt.t, 
Ragadinia, Zitt.t, and Pholidocladia, Hinde, and in the existent 
genus Discodermia, Bocage §, the outer surface of the sponge is 
covered with a layer of minute flat scale-like spicules, some with 
entire, others with lobate margins. In some a small shaft projects 
at right angles from the centre of the underside of the spicule, whilst 
* See Cat. Foss. Sponges Brit. Mus. pl. 32. f. 1. 
+ See Hinde, Foss. Sponge-spicules, t. 4. f. 35-46; and Cat. Foss. Sponges, 
pl. 20. f. 4a. 
t Cat. Foss. Sponges, pl. 20. f. 58. 
§ See Carter, Ann. & Mag. Nat. Hist. ser. 5, vol. vi. pl. 8. 
