Utica Slates, and the old Hudson River group, inseparably united
in what used to be called the "Blue Limestone Series."].

Of the _life_ of the Lower Silurian period we have record in
a vast number of fossils, showing that the seas of this period
were abundantly furnished with living denizens. We have, however,
in the meanwhile, no knowledge of the land-surfaces of the period.
We have therefore no means of speculating as to the nature of
the terrestrial animals of this ancient age, nor is anything
known with certainty of any land-plants which may have existed.
The only relics of vegetation upon which a positive opinion can
be expressed belong to the obscure group of the "Fucoids," and
are supposed to be the remains of sea-weeds. Some of the fossils
usually placed under this head are probably not of a vegetable
nature at all, but others (fig. 36) appear to be unquestionable
plants. The true affinities of these, however, are extremely
dubious. All that can be said is, that remains which appear to
be certainly vegetable, and which are most probably due to marine
plants, have been recognised nearly at the base of the Lower
Silurian (Arenig), and that they are found throughout the series
whenever suitable conditions recur.

The Protozoans appear to have flourished extensively in the Lower
Silurian seas, though to a large extent under forms which are
still little understood. We have here for the first time the
appearance of Foraminifera of the ordinary type--one of the most
interesting observations in this collection being that made by
Ehrenberg, who showed that the Lower Silurian sandstones of the
neighbourhood of St Petersburg contained casts in glauconite of
Foraminiferous shells, some of which are referable to the existing
genera _Rotalia_ and _Texularia_. True _Sponges_, belonging to
that section of the group in which the skeleton is calcareous,
are also not unknown, one of the most characteristic genera being
_Astylospongia_ (fig. 37). In this genus are included more or
less globular, often lobed sponges, which are believed not to
have been attached to foreign bodies. In the form here figured
there is a funnel-shaped cavity at the summit; and the entire
mass of the sponge is perforated, as in living examples, by a
system of canals which convey the sea-water to all parts of the
organism. The canals by which the sea-water gains entrance open
on the exterior of the sphere, and those by which it again escapes
from the