276 



HISTORICAL GEOLOGY. 



teur collectors and pressed as sea- weeds. If they be ex- 

 amined with a lens, they are seen to be composed of 



PIG. 174. FIG. 175. 



FIGS. 174, 175. Favositid and halysitid corals : 174. Columnaria alveolata. (After 

 Hall.) 175. Halysites catenulata. (After Hall.) 



hollow, branching stems, set on one or both sides with 

 hollow cups, each containing an animal which, if kept 

 undisturbed in sea-water, quickly spreads its thread-like 

 tentacles. These are the Hydrozoa of the present day 

 (Figs. 176, 177, 178). Now, in fine Silurian shales, which 



FIG. 176. FIG. 177. FHJ. 178. 



FIGS. 176-178. Living hydrozoa : 176 and 177. Surtiiluria. 178. Plinniilaria. 



were once fine mud, are found impressions of animals 

 probably similar to these. They are called Graptolites. 

 Whatever they be, they are easily recognized and wholly 



