existing Reptiles.  The tail of Archoeopteryx, therefore, is to be regarded as the permanent retention of an embryonic type of structure, or as an approximation to the characters of the Reptiles.  Another remarkable point in connection with Archoeopteryx, in which it differs from all known Birds, is, that the wing was furnished with two free claws.  From the presence of feathers, Archoeopteryx may be inferred to have been hot-blooded; and this character, taken along with the structure of the skeleton of the wing, may be held as sufficient to justify its being considered as belonging to the class of Birds.  In the structure of the tail, however, it is singularly Reptilian; and there is reason to believe that its jaws were furnished with teeth sunk in distinct sockets, as is the case in no existing Bird.  This conclusion, at any rate, is rendered highly probable by the recent discovery of “Toothed Birds” (Odonturnithes) in the Cretaceous rocks of North America.

[Illustration:  Fig. 183.—­Lower jaw of Amphitherium (Thylacotherium) Prevostii.  Stonesfield Slate (Great Oolite.)]

[Illustration:  Fig. 184.  Oolitic Mammals.—­1, Lower jaw and teeth of Phascolotherium, Stonesfield Slate; 2, Lower jaw and teeth of Amphitherium, Stonesfield Slate; 3, Lower jaw and teeth of Triconodon, Purbeck beds; 4, Lower jaw and teeth of Plagiaulax, Purbeck beds.  All the figures are of the natural size.]

The Mammals of the Jurassic period are known to us by a number of small forms which occur in the “Stonesfield Slate” (Great Oolite) and in the Purbeck beds (Upper Oolite).  The remains of these are almost exclusively separated halves of the lower jaw, and they indicate the existence during the Oolitic period in Europe of a number of small “Pouched animals” (Marsupials).  In the horizon of the Stonesfield Slate four genera of these little Quadrupeds have been described—­viz., Amphilestes, Amphitherium, Phascolotherium, and Stereognathus.  In Amphitherium (fig. 183), the molar teeth are furnished with small pointed eminences or “cusps;” and the animal was doubtless insectivorous.  By Professor Owen, the highest living authority on the subject, Amphitherium is believed to be a small Marsupial, most nearly allied to the living Banded Ant-eater (Myrmecobius) of Australia (fig. 158). Amphilestes and Phascolotherium (fig. 184) are also believed by the same distinguished anatomist and palaeontologist to have been insect-eating Marsupials, and the latter is supposed to find its nearest living ally in the Opossums (Didelphys) of America.  Lastly, the Stereognathus of the Stonesfield Slate is in a dubious position.  It may have been a Marsupial; but, upon the whole, Professor Owen is inclined to believe that it must have been a hoofed and herbivorous Quadruped belonging to the series of the higher Mammals (Placentalia).