[Footnote 12:  There is some difficulty about the precise nomenclature of this group.  It was originally called the “Hudson River Formation;” but this name is inappropriate, as rocks of this age hardly touch anywhere the actual Hudson River itself, the rocks so called formerly being now known to be of more ancient date.  There is also some want of propriety in the name of “Cincinnati Group,” since the rocks which are known under this name in the vicinity of Cincinnati itself are the representatives of the Trenton Limestone, 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 sponge open into the cup-shaped depression at the summit.