268 



LYELL'S ELEMENTS OF GEOLOGY. 



Lower Silurian Rocks, and their Fossils. 



divisible into similar groups. They are composed of large de- 

 posits of sandstone, which is sometimes found at the base of the 

 system, resting on gneiss and calcareous rocks, with orthocerata 

 and corals ; the chain-coral, (Fig. 278.) before mentioned, being 

 very conspicuous ; also fine bituminous shales containing grap- 

 tolites. (Fig. 280.) 



These bodies are supposed by Dr. Beck, of Copenhagen, to be 

 fossil zoophytes, related to the family of sea-pens, of which the 

 living animals inhabit mud and slimy sediment. 



Fig. 280. 



Graptotttcs, Linn. 



a. 6. Graptolites from Christiania, Norway. 

 c. d. Graptolites from the south of Sweden. 



In the limestones of Lake Michigan, in North America, and 

 other regions bordering the great Canadian lakes, chain-corals 

 and trilobites are also found, and from their fossils generally 

 they seem to belong to the Silurian period. They contain cer- 

 tain orthocerata, which have a very peculiar structure. The 

 siphuncle is very large, and has a tube running through its 

 whole length, from the outside of which radii set off in verticila- 

 tions extending to the inner wall of the syphon, these verticila- 

 tions corresponding in number to the chambers of the shell. 

 Mr. Stokes, who has described this division of orthoceratites, has 

 formed them into a distinct 'genus, for which he has adopted the 



Fig. 281. 



Actinoceras Simmsii, Stokes. 

 County of Down, Ireland. Length of original, 2 feet. 



name of Actinoceras, proposed by Professor Bronn.* The ac- 

 tinoceras was not known as a British fossil, until lately disco- 



* See Proceedings, Geol. Soc. 1838. 



