There are, indeed, not a few remarkable forms, as to the class position of which, whether they should be assigned to birds or reptiles, opinion was for a long time, and is in a few instances still, divided. It is, for example, only of late years that the fossil form Archæopteryx[36] (Fig. 4, [p. 19]) from the Solenhofen slates, has been definitely relegated to the former, but arguments against this disposal of it have been based upon the beak or jaws being furnished with true teeth, and the feather of the tail attached to a series of vertebræ, instead of a single flattened one as in birds. It appears to have been entirely plumed, and to have had a moderate power of flight.
Fig. 6.—Sivatherium (restored), from the Upper Miocene
deposits of the Siwalik Hills. (After Figuier.)
On the other hand, the Ornithopterus is only provisionally classed with reptiles, while the connection between the two classes is drawn still closer by the copious discovery of the birds from the Cretaceous formations of America, for which we are indebted to Professor Marsh.
The Lepidosiren, also, is placed mid-way between reptiles and fishes. Professor Owen and other eminent physiologists consider it a fish; Professor Bischoff and others, an amphibian reptile. It has a two-fold apparatus for respiration, partly aquatic, consisting of gills, and partly aerial, of true lungs.
So far, then, as abnormality of type is concerned, we have here instances quite as remarkable as those presented by most of the strange monsters with the creation of which mythological fancy has been credited.
Fig. 7.—Skeleton of Megatherium. (After Figuier.)
Among mammals I shall only refer to the Megatherium, which appears to have been created to burrow in the earth and to feed upon the roots of trees and shrubs, for which purpose every organ of its heavy frame was adapted. This Hercules among animals was as large as an elephant or rhinoceros of the largest species, and might well, as it has existed until a late date, have originated the myths, current among the Indians of South America, of a gigantic tunnelling or burrowing creature, incapable of supporting the light of day.[37]