CHAPTER V.

The Classifying Principle and its Uses.--Three Groups of Ichthyolites among the Organisms of the Lower Old Red Sandstone.--Peculiarities of the Third Group.--Its Varieties.--Description of the _Cheiracanthus_.--Of two unnamed Fossils of the same Order.--Microscopic Beauty of these ancient Fish.--Various Styles of Ornament which obtain among them.--The Molluscs of the Formation.--Remarkable chiefly for the Union of modern with ancient Forms which they exhibit.--Its Vegetables.--Importance and Interest of the Record which it furnishes, 79-94