87 
Spongida —Fossil Sponges. 
and thus only sponges with silicified or calcareous skeletons are 
Oulu in the rocks. The Silicispongise are by far the most 
mportant of these two divisions: their skeletons consist of 
nnu c spicules of silica of various forms, in some cases united 
| ler into a beautiful. mesh work, in others the spicules are 
_ose y held m position in the sarcode, and after the death of 
le sponge tbey are scattered over the sea-bottom. In this 
v aj- beds of rock are, in some instances, nearly entirely formed 
or the minute detached spicules of these sponges. 
The Silicispongiae are divided into four orders according to 
the form of their skeletal spicules (1) Monactinellidce , in 
Ulrich the spicules have but a single axis ; (2) Tetractinellidce, 
, T ‘ 1C ; 1 . the spicules have four rays or arms ; (3) Lithistidce , in 
which the spicules are four-rayed or irregular in form, and 
ultimately interwoven . together ; and (4) Hexactinellidce, in 
which the skeleton consists of spicules with six rays. As a rule 
entire sponges ot the two first -mentioned orders are rarely met 
with as fossils, though their detached spicules are very abundant 
more particularly in the Upper Greensand and the Upper Chalk.’ 
1 he greater number of fossil sponges belong to the Lithistidce 
and Hexactinellidce. 
With one or two exceptions fossil Calcisponges belong to the 
family of the Pharetrones. The spicules are mostly three or 
four-rayed, and they are united into a continuous fibrous net- 
work. 
Fossil sponges are first met with in Cambrian strata, the 
earliest known genus, Protospongia , belongs to the Hexacti- 
nel ida?. In the Silurian rocks the Lithistidse are represented 
by Astylospongia ' and Aulocopium ; and the peculiar families of 
the Peceptaculitidce and the Astrceospongid.ee occur here and in 
the Devonian. Hexactinellid sponges, allied to the recent 
Hyalonema , were numerous in Carboniferous strata, and are 
principally represented by detached spicules and by bands of 
elongated spicules, which served to anchor the sponges in the 
mud. ° 
A ith the exception of a small group of Calcisponges from 
the Triassic strata of St. Cassian, and from the Inferior Oolite 
of this country, fossil sponges are rarely met with until 
reaching the middle and upper Jura of Germany and Switzer- 
land, in which the Lithistidae and HexactineUidaa are very 
abundant. Calcisponges are numerous in the Lower Greensand 
of Faringdon, Berkshire; and in the Upper Greensand of the 
South of England, Lithistid sponges are largely developed, as 
well as spicules of Tetractinellidae and Monactinellidn?. Hexac- 
tinellid sponges distinguish certain zones of the gray Chalk and 
the Chalk Marl, and in the Upper Chalk representatives of all 
the groups of siliceous sponges are present. It is probable that 
the silica of the flints in the Upper Chalk is derived from the 
G-allery, 
No. 10. 
Fossil 
Sponges. 
Table-cases 
Nos. 11-15. 
Wall-cases, 
Nos. 7 and b 
