eS eee eee 
Fossils from a Croydon Garden. 89 
each other and grow together so intimately that the full-grown 
sponge has a firm, stony character, which enables it to resist 
disintegration. Mr. Muller’s specimen, which he supposed to be 
a radiolarian, furnished me with an important clue to the real 
structure of these fossils. 
In their perfect condition these sponges appear to have been 
provided with a thin outer layer which covered the stony resistant 
skeleton just described. This dermal layer is composed of small 
spicules of various forms, some simple rods, others with three or 
four rays (Pl. II., figs. 3,7). These spicules are only commingled 
or interfelted together, and not welded as the spicules of the 
sponge-body. Rarely is any definite arrangement shown, but in 
one instance they are concentrically disposed round a pore-like 
opening (Pl. IIL., fig. 10). Owing to the absence of any definite 
connection in the component spicules, this outer covering seems 
to have readily fallen to pieces after the death of the organism, 
and very rarely are portions of it preserved on the outside of the 
fossils. Out of a total of about 3000 specimens* I have only 
detected it in 18, some of which are figured (Pl. L., figs. 7,.8, 15, 
20, 25). In none of the Croydon specimens is it shown. 
These Chalk sponges were first referred to the genus Millepora 
by the late Professor John Phillips in 1829; since then they have 
been placed in various genera, according to the views held of their 
affinities, until at last, in 1878, Professor Steinmann proposed a 
distinct genus for their reception, and gave it the name of Poro- 
sphera. He considered the fossils, however, to be hydrozoa, and 
not sponges. Several species have been described; they are all 
closely allied, and most of them are represented in the collection 
exhibited. 
Porosphera globularis is the commonest form; .it is usually 
rounded like peas or marbles, but sometimes oval, loaf- or cushion- 
shaped, and without any distinctive base (Pl. I., figs. 1-10). 
P. nuciformis is typically pear-shaped, occasionally also melon- 
or loaf-shaped, with longitudinal ridges and shallow grooves which 
converge to the obtuse pole of the sponge (PI. I., figs. 11-18). 
P. Woodwardi is oval or rounded, with well-marked branching 
canals which converge to one or more points on the surface. It 
has a concave and rugose base (Pl. I., fig. 19). This species has 
only been found in the Grey Chalk of Dover and in Dorset. 
P, pileolus is thimble- or inverted cup-shaped, sometimes hemi- 
spherical, with a deeply concave, cup-shaped base, and thick walls 
(Pl. I., figs. 20-21). 
P. patelliformis is limpet-shaped, with peaked summit, a deeply 
* The large majority of these were collected by my friend Dr. A. W. Rowe, 
F.G.S., during his well-known researches in the zones of the White Chalk of 
the English coast, and I am greatly indebted to him for the opportunity of 
examining them. 
