Fig. 287. Hypothetical Map of Parts of North America and Europe in Silurian Time
Shaded areas, probable seas; broken lines, approximate shorelines
The western United States. So little is found of the rocks of the system west of the Missouri River that it is quite probable that the western part of the United States had for the most part emerged from the sea at the close of the Ordovician and remained land during the Silurian. At the same time the western land was perhaps connected with the eastern land of Appalachia across Arkansas and Mississippi; for toward the south the Silurian sediments indicate an approach to shore.
Life of the Silurian
In this brief sketch it is quite impossible to relate the many changes of species and genera during the Silurian.
| Fig. 288. A Compound Cup Coral | Fig. 289. A Simple Cup Coral |
Corals. Some of the more common types are familiarly known as cup corals, honeycomb corals, and chain corals. In the cup corals the most important feature is the development of radiating vertical partitions, or septa, in the cell of the polyp. Some of the cup corals grew in hemispherical colonies ([Fig. 288]), while many were separate individuals ([Fig. 289]), building a single conical, or horn-shaped cell, which sometimes reached the extreme size of a foot in length and two or three inches in diameter.