DR. G. J. HINDE ON RECEPTACULITID®. 829 
structure of recent and fossil siliceous sponges, makes it easier now 
to institute a comparison between them and the Receptaculitide 
than at the time when Mr. Billings wrote, and I now propose to 
show the extent of the resemblance. 
First as regards the outer form. Though this is such a variable 
feature in sponges that but little importance can be attached thereto, 
yet the fact that in outer form the various members of the Re- 
ceptaculitide are either platter- or cup-shaped, or conical, enclosing 
a central cavity with a summit aperture, is not without its signi- 
ficance when we consider that these are the commonest forms 
assumed by both recent and fossil sponges. Another feature too in 
which the Receptaculitide show a resemblance to undoubted Palseo- 
zoic sponges is the uniform absence of any point of attachment, 
so that the organism was perfectly free; thus contrasting with the 
usually attached Mesozoic and recent sponges. Ferd. Romer * has 
called particular attention to this fact in connection with the 
Paleozoic genera Astylospongia, Aulocopium, and Astreospongia, and 
also to the further circumstance, that the Paleozoic sponges do not 
form united colonies, but are distinct individuals, and this is the 
case also with all the members of the Receptaculitide. 
The only structural elements of the skeleton of the Receptaculitidee 
(if we except the inner layer of the genus Receptaculites itself) 
consist of the spicules, and these appear to me to be distinctly 
homologous with the spicules of hexactinellid sponges. The simi- 
larity between the four horizontal rays and the vertical rays of the 
spicules of this family and the same elements of the spicules of 
ordinary hexactinellid sponges is so close that it cannot fail to be 
recognized. The rays radiate at right angles to each other from a 
common centre; they gradually taper from the centre to their 
extremities (with the exception of the vertical ray in Receptaculites, 
which connects with the interior plate); and each ray is traversed 
by an axial canal which unites with the canals from the other rays 
in the central junction of the spicule. A resemblance (though not 
to hexactinellid spicules) is also presented by the peculiar neck-like 
contraction of the vertical spicular ray immediately beneath the 
horizontal rays in Receptaculites, to the spicules of Cretaceous 
examples of Goda, such as Geodia clavata, Hinde’, and G. coronata, 
Hinde, in which there is a similar contraction immediately beneath 
the head-rays. In Ischadites and Acanthochonia, five rays of the 
normal hexactinellid spicule are well developed and terminate freely 
in obtusely pointed extremities, whilst in Spherospongia and Recep- 
taculites only four rays are normal, the fifth or vertical ray in the 
former genus being apparently represented only by a short blunted 
process, whilst this ray in the latter genus is not free at its basal 
extremity, but organically attached to an inner plate. 
But though the analogy between the horizontal and vertical rays 
of the spicules and those of recognized hexactinellid sponges is 
readily apparent, yet this is not the case with the summit-plate or 
* Leth. Pal. Th. i. p. 306. 
+ Fossil Sponge-spicules from the Upper Chalk, pl. ii. 
