(3) Our third question leads to the inquiry in what modern or ancient marine animals we can find structures akin to those of our supposed Laurentian fossil. The first analogy which suggested itself to Sir W. Logan, and a very natural one, was that to the so-called layer-corals (Figs. 43 to 45) that abound in the Silurian, Ordovician, and Cambrian rocks, and which though undoubtedly fossil animals, have proved very difficult to interpret or to assign to any known group. At first vaguely associated with the true corals, they were subsequently regarded as probably of more simple character, and as gigantic Protozoa; and later strong reasons have been assigned for giving them an intermediate place, as allied to those curious communities of humble animals possessing simple stomachs and prehensile tentacles (Hydroids) which form some of the simpler corals (Millepores, etc.), and the crusts (Hydractiniæ) which cover dead shells and other bodies in the sea. When examined microscopically, however, they differ very much among themselves, and it may be that some of them were Hydroids and some Protozoa. [Illustration: Fig. 44.--_Structures of Stromatopora._ (_a_) Portion of oblique section, (_b_) Wall with pores, and coated with crystals of quartz, (_c_) Thickened portion of wall with canals, (_d_) Laminæ and pillars.] [Illustration: Fig. 45.--_Tubular Structure of Cœnostroma, Silurian._] The oldest that we at present know, and consequently the nearest in time to Eozoon, impress us rather with the latter affinity. They are the fossils of the genus Cryptozoon of Hall (Fig. 7[36]), which form great masses filling certain beds of Upper Cambrian age, and which, when sliced and studied microscopically, are found to consist of concentric thin laminæ filled in between with a porous mass of calcareous matter penetrated by an infinity of tortuous tubes. Forms of this kind have been traced downward into pre-Cambrian beds in Colorado, and as we shall find in New Brunswick, into the Upper Laurentian itself. [Footnote 36: See Figs. 7 and 7a, pp. 37, 38; also Fig. 8 and Microscopic slice, Fig. 59, at end.]