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Page:Popular Science Monthly Volume 24.djvu/627

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and in our own time the tendency is very fully displayed among a large number of forestine mammals.

During the secondary ages, however, it was the reptiles which took to thus developing a rudimentary flying-mechanism. Even at the present day there are some modern lizards, the "flying-dragons" of popular natural history, which possess a parachute arrangement of the front ribs, and are so enabled to jump lightly from branch to branch, somewhat in the same manner as the flying-squirrels. But this is an independent and comparatively late development of a flying apparatus among the reptiles, quite distinct in character from those which were in vogue among the real and much more terrible flying-dragons of the liassic and oölitic age. Far the most remarkable of these predecessors of the true birds were the pterodactyls whose bones we still find in our English cliffs at Lyme Regis and Whitby; creatures with a large reptilian head, fierce jaws set with sharp-pointed teeth, and fore-arms prolonged into a great projecting finger so as to support a membranous wing or fold of skin, somewhat analogous to that of the bats. The pterodactyls do not stand anywhere in the regular line of descent toward the true birds; but they are interesting as showing that a general tendency then existed among the higher reptiles toward the development of a flying organ. In these frightful dragons, the organ of flight is formed by an immense prolongation of the last finger on each fore-leg, to a length about as great as that of the rest of the leg all put together. Between this long bony finger and the hind-leg there stretched, in all probability, a featherless wing like a bat's, by means of which the pterodactyl darted through the air and pounced down upon its cowering victims. As in birds, the bones were made very light, and filled with air instead of marrow; and all the other indications of the skeleton show that the creatures were specially designed for the function of flight. Imagine a cross between a vulture and a crocodile, and you have something like a vague mental picture of a pterodactyl.

But at the very time when the terrestrial reptilian type was branching out in one direction toward the ancestors of the pterodactyls, it was branching out in another direction toward the ancestors of the true birds. In the curious lithographic slate of Solenhofen we have preserved for us a great number of fossil forms with an extraordinary degree of perfection; and among these are several which help us on greatly from the reptilian to the avian structure. The lithographic slate is a member of the upper oölitic formation, and it is worked, as its name implies, for the purpose of producing stones for the process of lithography. But the same properties which make the slate in its present condition take so readily the impress of a letter or a sketch made it in its earlier condition take the impress of the various organisms imbedded as they fell in its soft mud. Even the forms and petals of early flowers washed down by floods into the half-formed mud-bank have been thus preserved for us with wonderful minute-