The Descent of the Ammonites 197 
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 con- 
cealed 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 goniatites having zigzag, angulated sutures. 
Late in the succeeding Carboniferous period appear shells with a 
truly ammonoid complexity of sutures, and in the Permian their 
number and variety cause them to form a striking element of the 
marine faunas. It is in the Mesozoic era, however, that these shells 
attain their full development; increasing enormously in the Triassic, 
they culminate in the Jurassic in the number of families, genera and 
