In the Silurian and Devonian ages, these Stromatoporæ evidently carried out the same function as the Eozoon in the Laurentian. Winchell tells us that in Michigan and Ohio single specimens can be found several feet in diameter, and that they constitute the mass of considerable beds of limestone. I have myself seen in Canada specimens a foot in diameter, with a great number of laminæ. Lindberg[AK] has given a most vivid account of their occurrence in the Isle of Gothland. He says that they form beds of large irregular discs and balls, attaining a thickness of five Swedish feet, and traceable for miles along the coast, and the individual balls are sometimes a yard in diameter. In some of them the structure is beautifully preserved. In others, or in parts of them, it is reduced to a mass of crystalline limestone. This species is of the Cœnostroma type, and is regarded by Lindberg as a coral, though he admits its low type and resemblance to Protozoa. Its continuous calcareous skeleton he rightly regards as fatal to its claim to be a true sponge. Such a fossil, differing as it does in minute points of structure from Eozoon, is nevertheless probably allied to it in no very distant way, and a successor to its limestone-making function. Those which most nearly approach to Foraminifera are those with thick and solid calcareous laminæ, and with a radiating canal system; and one of the most Eozoon-like I have seen, is a specimen of the undescribed species already mentioned from the Guelph (Upper Silurian) limestone of Ontario, collected by Mr. Weston, and now in the Museum of the Geological Survey. I have attempted to represent its structures in fig. 44. [Footnote AK: _Transactions of Swedish Academy_, 1870.] [Illustration: Fig. 46. _Receptaculites, restored._ (_After Billings._) (_a._) Aperture. (_b._) Inner wall. (_c._) Outer wall. (_n._) Nucleus, or primary chamber. (_v._) Internal cavity.] [Illustration: Fig. 47. _Diagram of Wall and Tubes of Receptaculites._ (_After Billings._) (_b._) Inner wall. (_c._) Outer wall. (_d._) Section of plates. (_e._) Pore of inner wall. (_f._) Canal of inner wall. (_g._) Radial stolon. (_h._) Cyclical stolon. (_k._) Suture of plates of outer wall.] [Illustration: Fig. 48. _Receptaculites, Inner Surface of Outer Wall with the Stolons remaining on its Surface._ (_After Billings._)]