middle line, such as exists in all the flying animals.  Its breast-bone, on the contrary, is thin and flat, like that of the present Sea-Turtle; and if it moved through the water by the help of its long flappers, as the Sea-Turtle does now, it could well dispense with that powerful construction of the breast-bone so essential to all animals which fly through the air.  Again, the powerful teeth, long and conical, placed at considerable intervals in the jaw, constitute a feature common to all predaceous aquatic animals, and would seem to have been utterly useless in a flying animal at that time, since there were no aerial beings of any size to prey upon.  The Dragon-Flies found in the same deposits with the Pterodactylus were certainly not a game requiring so powerful a battery of attack.

The Fishes of the Jurassic sea were exceedingly numerous, but were all of the Ganoid and Selachian tribes.  It would weary the reader, were I to introduce here any detailed description of them, but they were as numerous and varied as those living in our present waters.  There was the Hybodus, with the marked furrows on the spines and the strong hooks along their margin,—­the huge Chimera, with its long whip, its curved bone over the back, and its parrot-like bill,—­the Lepidotus, with its large square scales, its large head, its numerous rows of teeth, one within another, forming a powerful grinding apparatus,—­the Microdon, with its round, flat body, its jaw paved with small grinding teeth,—­the swift Aspidorhynchus, with its long, slender body and massive tail, enabling it to strike the water powerfully and dart forward with great rapidity.  There were also a host of small Fishes, comparing with those above mentioned as our Perch, Herring, Smelts, etc., compare with our larger Fishes; but, whatever their size or form, all the Fishes of those days had the same hard scales fitting to each other by hooks, instead of the thin membranous scales overlapping each other at the edge, like the common Fishes of more modern times.  The smaller Fishes, no doubt, afforded food to the larger ones, and to the aquatic Reptiles.  Indeed, in parts of the intestines of the Ichthyosauri, and in their petrified excrements, have been found the scales and teeth of these smaller Fishes perfectly preserved.  It is amazing that we can learn so much of the habits of life of these past creatures, and know even what was the food of animals existing countless ages before man was created.

There are traces of Mammalia in the Jurassic deposits, but they were of those inferior kinds known now as Marsupials, and no complete specimens have yet been found.