Fossils from a Croydon Garden. 91 
The unexpected discovery of the recent Japanese sponge and 
the Tertiary form from Australia with structures so closely related 
to those of the Chalk Porosphera clearly establish that this latter 
genus is a calcisponge with the skeleton spicules fused together. 
Dr. Rauff has placed these sponges in a separate order, the 
Lithonina.* 
There is one feature of more general interest in connection 
with these Chalk sponges which may be mentioned. Many of 
the rounded and pear-shaped specimens have a cylindrical hole 
or perforation, which in some cases extends only for a short 
distance and terminates blindly, but more frequently it passes 
quite through the specimen, so that it is a genuine bead (PI. I., 
fig. 1). At one time these perforations were thought to be arti- 
ficial, and due to human agency ; but it is now generally admitted 
that they are natural, and probably arise from the sponge after 
having passed through the early, mobile stage of its existence, 
fixing itself on and growing round the stem of a seaweed or some 
other marine organism not capable of preservation in the fossil 
state. On the decay and disappearance of the supporting body 
the more resistant sponge would be left with the hollow cast, 
which subsequently becomes filled with the soft chalky matrix 
as we now find it.” 
Both in this country and in the North of France these sponge- 
beads have been found in association with the remains of the 
‘*River Drift” folk; and it has been surmised by Sir John 
Evans, the late Sir Charles Lyell, and other writers, that these 
prehistoric inhabitants may have used them for personal adorn- 
ment. To show their suitableness for this purpose I have strung 
those picked up in my garden—seventy-seven in number—and it 
will be seen that the necklace which they form might well prove 
attractive to a primitive race. 
In addition to Porosphera, described above, the same beds of 
Chalk in this part of Croydon contain another kind of sponge so 
generally similar in form and size to Porosphera, that the two 
are frequently confounded with each other. The form referred to 
much resembles a small weathered flint pebble; some specimens, 
when broken cpen, are found to be of nearly solid flint, with 
slight traces of spicular structure; in others there is a com- 
paratively thin outer layer of flint completely enclosing a loose 
central core or kernel of porous flint, which is the cast of the 
sponge. These sponges have a siliceous skeleton of irregular, 
four-rayed warty spicules connected into a mesh-work; thus very 
distinct from the skeleton of Porosphera. The late Prof. v. Zittel 
placed them in the Lithistid genus Plinthosella. Proportionally 
_ they are less numerous than Porosphera, but 225 specimens have 
been collected in the same garden area. 
* Paleontographica, Bd. 40, 1893, pp. 203, 204. - 
