198 



GEOLOGY. 



Fig. 10T. 



latter have also peculiar arms, from which they are com- 

 monly called ~Brachiopods, from brachion, arm, and pous. 

 The valves are symmetrical, as seen in 1, 2, 3, 4, and 5, 

 Fig. 106, which are all Brachiopods. The Brachiopods 

 were much more abundant in the Silurian age than the 

 common bivalves, but the reverse is the case at the pres- 

 ent time. The small shell, Lingula, 1, is some- 

 times found in rocks in such abundance as to 

 give them their name, as Lingula flags and Lin- 

 gula grits. The Orthoceras, 7, is a chambered 

 shell, like the pearly Nautilus of our day, or the 

 Lituites in the figure, but as if unrolled, so as to 

 be straight. Some specimens are ten or fifteen 

 feet in length and a foot in diameter. These 

 were the monster mollusks of the Molluscan 

 age. In Fig. 108 is represented the internal 

 arrangement of the shells of these Orthocera- 

 tites. The Cephalopods, with their chambered 

 cells, were very numerous in the Silurian and 

 other ancient ages, but at the present time there 

 are about half a dozen living species, and these 

 belong to the genus Nautilus. In Fig. 109 we have a 

 representation of a remarkable shell of a cephalopod from 

 the Silurian rocks of Russia. 



287. Trilobites. The family of Trilobites, of which 

 there is not a species now in existence, furnished the 



Fig. 108. 



