CHAPTER IV 
MORE ABOUT FOSSILS 
_ Tue last chapter conveniently concluded our study 
of fossils with the last sub-kingdom of the inverte- 
brate or backboneless animals, the Mollusca. We 
shall now proceed to deal with the eighth great sub- 
kingdom, the VERTEBRATA, or Cordaia, the members 
of which can be easily recognized by their possession 
of backbones and nerve-cords. The Vertebrata are 
divided into five leading classes : 
1. Fishes.—Animals which breathe by gills, and 
are cold-blooded. They have their bodies covered 
with scales, and their limbs, when present, appear in 
the form of fins. Fossil fish are found in Upper 
Silurian strata, and they constitute the earliest 
traces of vertebrate life-forms which have been dis- 
covered. Some Old Red Sandstone rocks of the 
Devonian Period contain great numbers of fish 
remains. In some flagstones and sandstones of that 
age fossil fish are found piled up on each other, and 
in a well-preserved condition. It would seem as if 
shoals of the fish had met with a sudden death, and 
their bodies had been covered by sediments very 
quickly, before they could decay or be destroyed. 
The celebrated Hugh Miller has told us about his 
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