160 



THE DAWN OP LIFE. 



sometimes preserved merely by infiltration with cal- 

 cite or dolomite, and in this case it is most difficult to 

 make out their minute structures. Often they appear 

 merely as concentrically laminated masses which, but 



FIG. 45. CcenostromaGuelph Limestone, Upper Silurian, from a 

 specimen collected by Mr. Weston, showing the canals. 



(a.) Surface with canals, natural size. (&.) Vertical section, natural size, (c.) 

 The same magnified, showing canals and laminae. 



for their mode of occurrence, might be regarded as 

 mere concretions. In other cases the cell-walls and 

 pillars are perfectly silicified, and then they form beau- 

 tiful microscopic objects, especially when decalcified 

 with an acid. In still other cases, they are preserved 

 like Eozoon, the walls being calcareous and the cham- 

 bers filled with silica. In this state when weathered 

 or decalcified they are remarkably like Eozoon, but I 

 have not met with any having their minute pores and 

 tubes so well preserved as in some of the Laurentian 

 fossils. In many of them, however, the growth and 

 overlapping of the successive amoeba-like coats of sar- 

 code can be beautifully seen, exactly as on the surface 

 of a decalcified piece of Eozoon. Those in my collec- 

 tion which most nearly resemble the Laurentian speci- 



