THE SILURIAN PERIOD 



355 



FIG. 367. A coiled Silurian ceph- 

 alopod (Phragmoceras). 



the fossils already discovered show that, even before the Silu- 

 rian period, this group had become divided into its constituent 



classes, such as insects, scor- 

 pions, and others. 



Relations with Europe. A 



fauna very similar to that just 



described lived in a sea which 



occupied the site of England 



and the Baltic region during 



the same time. It is thought 



that the route of intermigra- 



tion between the two conti- 

 nents lay along a shallow-water 



tract which extended up 



through Canada and Alaska 



and perhaps even the polar 



regions. So easy was the 



communication along this path that peculiar Swedish corals 

 and trilobites found their way over to 

 Iowa, and crinoids characteristic of the 

 United States became residents also of 

 England. 



Silurian deserts. The quiet continu- 

 ance of these broad epicontinental seas 

 was interrupted in both continents by 

 changes of far-reaching importance. The 

 deposition of limestone in eastern United 



States gradually ceased, and, in some _ 



areas, if not in all, this was occasioned 



by the emergence of the sea bottom into 



a low-lying land. In the West at this 



time much of the region from Montana 



south westward remained under water. FlG 369 _ A Si i ur j an 

 In the East the Niagara limestone is brachiopod (Ortho- 



frequently found lying unconformably be- ? a f v s arie ^ s e 



neath the later deposits. In the districts long hinge line. 



FIG. 368. A small 

 pointed brachiopod 

 (Rhynchotreta) char- 

 acteristic of the Silu- 

 rian rocks. 



