ent turtles; and the oldest known Pterosaurs,
the flying dragons of the Jurassic, are already fully differentiated.
There is, however, no ground for discouragement in this, for the
progress of discovery has been so rapid of late years, and our knowledge
of Mesozoic life has increased with such leaps and bounds, that there is
every reason to expect a solution of many of the outstanding problems in
the near future.

Passing over the lower vertebrates, for lack of space to give them
any adequate consideration, we may briefly take up the record of
invertebrate life. From the overwhelming mass of material it is
difficult to make a representative selection and even more difficult
to state the facts intelligibly without the use of unduly technical
language and without the aid of illustrations.

Several groups of the Mollusca, or shell-fish, yield very full and
convincing evidence of their descent from earlier and simpler forms,
and of these none is of greater interest than the Ammonites, an extinct
order of the cephalopoda. The nearest living ally of the ammonites is
the pearly nautilus, the other existing cephalopods, such as the squids,
cuttle-fish, octopus, etc., are much more distantly related. Like the
nautilus, the ammonites all possess a coiled and chambered shell, but
their especial characteristic is the complexity of the "sutures." By
sutures is meant the edges of the transverse partitions, or septa, where
these join the shell-wall, and their complexity in the fully developed
genera is extraordinary, forming patterns like the most elaborate
oak-leaf embroidery, while in the nautiloids the sutures form simple
curves. In the rocks of the Mesozoic era, wherever conditions of
preservation are favourable, these beautiful shells are stored in
countless multitudes, of an incredible variety of form, size and
ornamentation, as is shown by the fact that nearly 5000 species have
already been described. The ammonites are particularly well adapted for
phylogenetic studies, because, by removing the successive whorls of the
coiled shell, the individual development may be followed back in inverse
order, to the microscopic "protoconch," or embryonic shell, which lies
concealed in the middle of the coil. Thus the valuable aid of embryology
is obtained in determining relationships.

The descent of the ammonites, taken as a group, is simple and clear;
they arose as a branch of the nautiloids in the lower Devonian, the
shells known as