Of all the fossils of the Silurian rocks those which most resemble Eozoon are the _Stromatoporæ_, or "layer-corals," whose resemblance to the old Laurentian fossil at once struck Sir William Logan; and these occur in the earliest great oceanic limestones which succeed the Primordial period, those of the Trenton group, in the Siluro-Cambrian. From this they extend upward as far as the Devonian, appearing everywhere in the limestones, and themselves often constituting large masses of calcareous rock. Our figure (fig. 42) shows a small example of one of these fossils; and when sawn asunder or broken across and weathered, they precisely resemble Eozoon in general appearance, especially when, as sometimes happens, their cell-walls have been silicified. [Illustration: Fig. 42. _Stromatopora rugosa, Hall--Lower Silurian, Canada._ (_After Billings._) The specimen is of smaller size than usual, and is silicified. It is probably inverted in position, and the concentric marks on the outer surface are due to concretions of silica.] There are, however, different types of these fossils. The most common, the Stromatoporæ properly so called, consist of concentric layers of calcareous matter attached to each other by pillar-like processes, which, as well as the layers, are made up of little threads of limestone netted together, or radiating from the tops and bottoms of the pillars, and forming a very porous substance. Though they have been regarded as corals by some, they are more generally believed to be Protozoa; but whether more nearly allied to sponges or to Foraminifera may admit of doubt. Some of the more porous kinds are not very dissimilar from calcareous sponges, but they generally want true oscula and pores, and seem better adapted to shield the gelatinous body of a Foraminifer projecting pseudopods in search of food, than that of a sponge, living by the introduction of currents of water. Many of the denser kinds, however, have their calcareous floors so solid that they must be regarded as much more nearly akin to Foraminifers, and some of them have the same irregular inosculation of these floors observed in Eozoon. Figs. 43, A to D, show portions of species of this description, in which the resemblance to Eozoon in structure and arrangement of parts is not remote. [Illustration: Fig. 43. _Structures of Stromatopora._ (_a._) Portion of an oblique section magnified, showing laminæ and columns. (_b._) Portion of wall with pores, and crusted on both sides with quartz crystals. (_c._) Thickened portion of wall with canals. (_d._) Portion of another specimen, showing irregular laminæ and pillars.]