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54 



THE CHAIN OF LIFE. 



are represented in Fig. 37. The little teeth on the sides of 

 these were cells, inhabited probably by polyps, like those repre- 

 sented in the modern Sertularia in Fig. 36. Some of them 

 were probably attached to the bottom. In others the branches 

 radiated from a central film, as in Fig. 38, which may have 

 been a hollow vesicle or float, enabling them to live at the 

 surface of the water. These Graptolites are specially character- 

 istic of the Upper Cambrian and Lower Silurian. The netted 



Fig. 36. —Group of modem Hydroids allied to Graptolites. Magnified, and natural size. 

 a, Sertularia, b, Tubttlaria. c, Campaniilaria. 



ones {Dictyonema), as may be seen from Figs. 34 and 35, came 

 in before the close of the Cambrian, and continue unchanged 

 to the Upper Silurian, where they disappear. The branching 

 forms, seen in Fig. 37, have scarcely so great a range. They 

 thus form most certain marks of the period to which they 

 belong, and being oceanic and prcbably floaters, they diffused 

 ^themselves so rapidly that they appear to indicate the same 

 geological time in countries so widely separated as Europe, 



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