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. [Illustration: Fig. 37.--_Astylospongia proemorsa_, cut vertically so as to exhibit the canal-system in the interior. Lower Silurian, Tennessee. (After Ferdinand Roemer.)] The most abundant, and at the same time the least understood, of Lower Silurian Protozoans belong, however, to the genera _Stromatopora_ and _Receptaculites_, the structure of which can merely be alluded to here. The specimens of _Stromatopora_ (fig. 38) occur as hemispherical, pear-shaped, globular, or irregular masses, often of very considerable size, and sometimes demonstrably attached to foreign bodies. In their structure these masses consist of numerous thin calcareous laminæ, usually arranged concentrically, and separated by narrow interspaces. These interspaces are generally crossed by numerous vertical calcareous pillars, giving the vertical section of the fossil a lattice-like appearance. There are also usually minute pores in the concentric laminæ, by which the successive interspaces are placed in communication; and sometimes the surface presents large rounded openings, which appear to correspond with the water-canals of the Sponges. Upon the whole, though presenting some curious affinities to the calcareous Sponges, _Stromatopora_ is perhaps more properly regarded as a gigantic _Foraminifer_. If this view be correct, it is of special interest as being probably the nearest ally of _Eozoön_, the general appearance of the two being strikingly similar, though their minute structure is not at all the same. Lastly, in the fossils known as _Receptaculites_ and _Ischadites_ we are also presented with certain singular Lower Silurian Protozoans, which may with great probability be regarded as gigantic _Foraminifera_.