438 
ELEMENTS OF GEOLOGY. 
Fig^493. ognized. The first two genera are common 
to this and all the after periods; the third 
has been found in the Upper Silurian, but is 
not known above the Carboniferous • strata; 
3 ditini. Moiiutaiu the fourth (Fig. 493) is characteristic of the 
Limestone. Mountain Limestone in the United States, 
Arctic America, Russia, and Asia Minor, but is also known 
in the Permian. 
