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.
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.
These fossils, however, show no very distinct canal system or supplemental skeleton, but this also appears in those forms which have been called Caunopora or Cœnostroma. In these the plates are traversed by tubes, or groups of tubes, which in each successive floor give out radiating and branching canals exactly like those of Eozoon, though more regularly arranged; and if we had specimens with the canals infiltrated with glauconite or serpentine, the resemblance would be perfect. When, as in figs. 44 and 45 A, these canals are seen on the abraded surface, they appear as little grooves arranged in stars, which resemble the radiating plates of corals, but this resemblance is altogether superficial, and I have no doubt that they are really foraminiferal organisms. This will appear more distinctly from the sections in [fig. 45] B, C, which represents an undescribed species recently found by Mr. Weston, in the Upper Silurian limestone of Ontario.