4 THE SNAKES OF SOUTH AFRICA. 
nervous systems and brains than the reptiles. In the very early 
strata of sedimentary rocks, dating back millions of years ago, 
and known in science as the Ordovician System, we find the fossil 
remains of very low forms of backboneless creatures, such as 
worms, jelly-fish, molluscs, etc. Then, as we ascend into the 
strata of rocks known as the Silurian System, we find more 
highly-evolved creatures, such as fish without bony backbones, 
Fic. 1.—These are Flying Reptiles—the ancestors of birds. They lived millions of years ago. 
Their fossil remains have been found preserved in the oolitic rocks. Some of them measured 
18 feet across the expanded wings. (From ‘‘ Extinct Animals ’”’ by Sir Ray Lankester. 
Messrs. Constable & Co., London.) 
or backbones composed of cartilaginous matter. These are 
known as the Silurian fishes, of which our present-day Shark 
Family is an example. 
As we slowly ascend we find the fishes gradually becoming 
more highly developed or specialized ; and when we reach what 
is known as the Devonian System, or, in other words, the time 
in the earth’s history many millions of years ago when the old 
Red Sandstone was deposited as sediment and formed into hard 
rock, we discover the higher order of scaled fishes with real true 
backbones of hard bony matter. These were the first to evolve 
into what we term vertebrata, z.e. forms of life with backbones. 
