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North America； are so extremely rare and so very imperfectly known； that they give little help in determining the descent of the class； but； on the other hand； certain reptilian orders of the Permian period； especially well represented in South Africa； display so many and such close approximations to mammalian structure； as strongly to suggest a genetic relationship。 It is difficult to believe that all those likenesses should have been independently acquired and are without phylogenetic significance。
Birds are comparatively rare as fossils and we should therefore look in vain among them for any such long and closely knit series as the mammals display in abundance。 Nevertheless； a few extremely fortunate discoveries have made it practically certain that birds are descended from reptiles； of which they represent a highly specialised branch。 The most ancient representative of this class is the extraordinary genus Archaeopteryx from the upper Jurassic of Bavaria； which； though an unmistakable bird； retains so many reptilian structures and characteristics as to make its derivation plain。 Not to linger over anatomical minutiae； it may suffice to mention the absence of a horny beak； which is replaced by numerous true teeth； and the long lizard…like tail； which is made up of numerous distinct vertebrae； each with a pair of quill…like feathers attached to it。 Birds with teeth are also found in the Cretaceous； though in most other respects the birds of that period had attained a substantially modern structure。 Concerning the interrelations of the various orders and families of birds； palaeontology has as yet little to tell us。
The life of the Mesozoic era was characterised by an astonishing number and variety of reptiles； which were adapted to every mode of life； and dominated the air； the sea and the land； and many of which were of colossal proportions。 Owing to the conditions of preservation which obtained during the Mesozoic period； the history of the reptiles is a broken and interrupted one； so that we can make out many short series； rather than any one of considerable length。 While the relations of several reptilian orders can be satisfactorily determined； others still baffle us entirely； making their first known appearance in a fully differentiated state。 We can trace the descent of the sea…dragons； the Ichthyosaurs and Plesiosaurs； from terrestrial ancestors； but the most ancient turtles yet discovered show us no closer approximation to any other order than do the recent 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 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 species； in the complexity of the sutures； and in the variety of shell…ornamentation。 A slow decline begins in the Cretaceous； ending in the complete extinction of the whole group at the end of that period。 As a final phase in the history of the ammonites； there appear many so…called 〃abnormal〃 genera； in which the shell is irregularly coiled； or more or less uncoiled； in some forms becoming actually straight。 It is interesting to observe that some of these genera are not natural groups； but are 〃polyphyletic；〃 i。e。 are each derived from several distinct ancestral genera； which have undergone a similar kind of degeneration。
In the huge assembly of ammonites it is not yet possible to arrange all the forms in a truly natural classification； which shall express the various interrelations of the genera； yet several beautiful series have already been determined。 In these series the individual development of the later general shows transitory stages which are permanent in antecedent genera。 To give a mere catalogue of names without figures would not make these series more intelligible。
The Brachiopoda； or 〃lamp…shells；〃 are a phylum of which comparatively few survive to the present day； their shells have a superficial likeness to those of the bivalved Mollusca； but are not homologous with the latter； and the phylum is really very distinct from the molluscs。 While greatly reduced now； these animals were incredibly abundant throughout the Palaeozoic era； great masses of limestone being often composed almost exclusively of their shells； and their variety is in keeping with their individual abundance。 As in the case of the ammonites； the problem is to arrange this great multitude of forms in an orderly array that shall express the ramifications of the group according to a genetic system。