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 



