One of the most remarkable advances which Jurassic life has to show consists in the first appearance of the birds. As yet, only a single kind of Jurassic bird has been found, and that in the Solenhofen limestones. This, the most ancient known bird, is called Archaeopteryx (Fig. 297), and has many points of resemblance to the reptiles, and many characters which recur only in the embryos of modern birds. The peculiarities which strike one at the first glance are the head and tail; there was no horny beak, but the jaws are set with a row of small teeth, while the tail is very • long, composed of separate vertebrae, and with a pair of quillfeathers attached to each joint. The wing is constructed on the same plan as that of a modern bird, but is decidedly more primitive. The four fingers are all free (in recent birds two of the three fingers are fused together); they have the same number of joints as in the lizards, and are all provided with claws. The plumage is thoroughly bird-like in character, but is peculiar in the presence of quill-feathers on the legs, and apparently also in the absence of contour feathers from the head, neck, and much of the body, leaving those parts naked.
Fig. 297. - Restoration of Archceopteryx lithographica v. Meyer. (Andreae).
This very extraordinary creature was, then, a true bird, but had retained many features of its reptilian ancestry, and shows us that those ancestors have still to be sought in the Trias or even the Permian.
The mammals of the Jurassic are still very rare and imperfectly known, and have been found in only a few places. How many mammalian genera should be referred to the Jurassic will depend upon where the somewhat arbitrary line is drawn, which separates that system from the Cretaceous.
The Multituberculata are regarded as belonging to the most primitive kind of Mammals, the Monotremata, at present represented only by the duckbilled Mole (Ornithorhynchus) and Spiny Ant-Eater (Echidna) of Australia, animals which, though warm-blooded and suckling their young, reproduce by laying an egg. Of the Multituber-culates the most prominent Jurassic representatives are the English genus Plagiaulax, from the Purbeck, and the American genera Ctenacodon and Allodon, from the Morrison of Wyoming. In another group, which may be related to the Marsupials, the teeth are simpler and more numerous; examples of this group are the Purbeckian genera, Stylodon, and Triconodon, and the Morrison genera, Dryolestes and Dicrocynodon. Thus, at the very end of the Jurassic, the mammals are still tiny, insignificant creatures, which play but a very subordinate role in the luxuriant terrestrial life of the period.