a single pair of quill-feathers. In the
flying Birds of the present day, as before mentioned, the terminal
vertebrae 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 vertebrae, 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.]

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 "cus