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 laminae, 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 laminae, 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 _Eozooen_, 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_. Their
structure is very complex; but fragments are easily recognised
by the fact that the exterior is covered with numerous rhomboidal
calcareous plates, closely fitting together, and arranged in
peculiar intersecting curves, presenting very much the appearance
of the engine-turned case of a watch.

[Illustration: Fig. 38.--A small and perfect specimen of
_Stromatopora rugosa_, of the natural size, from the Trenton
Limestone of Canada. (After Billings.)]

Passing next to the sub-kingdom of _Coelenterate_ animals (Zoophytes,
Corals, &c.), we find that this great group, almost or wholly
absent in the Cambrian, is represented in Lower Silurian deposits
by a great number o