If we attempt to trace the Rhizopods onward from the Middle Laurentian, we are met with a great hiatus in the Upper Laurentian. The species _Eozoon Bavaricum_ has, however, been found in rocks apparently of Huronian age; but this is the last known appearance of _Eozoon_, properly so-called. In the Cambrian or Siluro-Cambrian, however, we meet with many gigantic Protozoa, more especially those known as _Stromatopora_, _Archæocyathus_, _Receptaculites_, and _Cryptozoon_. [Illustration: FIG. 22.—_Stromatopora concentrica._—After Hall. _a_, Section of the same, magnified. _b_, Small portion highly magnified, showing laminæ and pillars.] The typical Stromatoporæ, or Layer-corals, consist, like _Eozoon_, of concentric layers, connected by numerous pillars, which are often, though not always, more definite and regular than in the Laurentian fossil. The laminæ are perforated, but more coarsely than in _Eozoon_, and they are often thickened with supplemental deposit which, in some of the forms, presents canals radiating from vertical tubes or bundles of tubes penetrating the mass (Figs. 22, 23). The mode of growth of _Stromatopora_ must have closely resembled that of _Eozoon_, and the forms produced are so similar that it is often quite impossible to distinguish them by the naked eye. Like _Eozoon_, they form the substance of important limestones, and single masses are sometimes found as much as three feet in diameter. The Stromatoporæ extend from the Upper Cambrian to the Devonian inclusive. In the Carboniferous they are continued by smaller and more regular organisms of the genus _Loftusia_,[5] and this genus seems to extend without marked change up to the Eocene Tertiary. Recent students of the Stromatoporæ seem disposed to promote them from the province of Protozoa to that of the Hydroids.[6] The reasons for this seem cogent in the case of some of the forms, but in my judgment fail in others, more especially in the older forms. It may ultimately be found that the group as now held includes very different types of structure. In modern times I know of no nearer representative than the animal whose skeleton often adheres in red encrusting patches to our specimens of corals, and which is known as _Polytrema_. In general structure it is not very far from being a very degenerate kind of _Stromatopora_. [Illustration: FIG. 23.—_Caunopora planulata._ Showing the radiating canals on a weathered surface. Devonian.—After Hall.]