The great class of the Birds, as we have seen, is represented in rocks earlier than the Oolites simply by the not absolutely certain evidence of the three-toed footprints of the Connecticut Trias. In the Lithographic Slate of Solenhofen (Middle Oolite), there has been discovered, however, the at present unique skeleton of a Bird well known under the name of the _Archoeopteryx macrura_ (figs. 181, 182). The only known specimen--now in the British Museum--unfortunately does not exhibit the skull; but the fine-grained matrix has preserved a number of the other bones of the skeleton, along with the impressions of the tail and wing feathers. From these remains we know that _Archoeopteryx_ differed in some remarkable peculiarities of its structure from all existing members of the class of Birds. This extraordinary Bird (fig. 182) appears to have been about as big as a Rook--the tail being long and extremely slender, and composed of separate vertebræ, each of which supports a single pair of quill-feathers. In the flying Birds of the present day, as before mentioned, the terminal vertebræ of the tail are amalgamated to form a single bone ("ploughshare-bone"), which supports a cluster of tail-feathers; and the tail itself is short. In the embryos of existing Birds the tail is long, and is made up of separate vertebræ, and the same character is observed in many 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.]