Xttl PHYLUM CHORDATA 405 



is longer than the sixth, and the proximal portion of the tarso- 

 metatarsus is feathered. 



3. GENERAL ORGANISATION. 



In respect of range of structural variations, the entire class of 

 Birds is hardly the equivalent of a single order of Reptiles. Among 

 existing Birds, the Emu and the Raven, which may be said to stand 

 at opposite ends of the series, present nothing like the anatomical 

 differences to be found between a common Lizard and a Chamaeleon, 

 or between a Turtle and a Tortoise. Hence in dividing the class 

 into orders we find none of those striking distinctive characters 

 which separate the orders of Fishes, Amphibia, and Reptiles, but 

 have to be content with characters which in other groups would be 

 considered insignificant, such as details in the structure of the 

 skull and sternum, in the arrangement of the muscles of the wing 

 and leg, in the form of the foot, and in the peculiarities of the 

 newly-hatched young. It is for this reason that in the preceding 

 classification no diagnoses of the orders are given : to define them 

 adequately would involve a degree of anatomical detail quite beyond 

 the scope of the present work. 



The differences between the two avian sub-classes, the Archae- 

 ornithes and the Neornithes, are, however, of a far more fundamental 

 nature ; and as Archaeopteryx, the sole representative of the first of 

 these groups, is a unique form, and perhaps the best example of 

 an undoubted link between two classes Reptiles and Birds it 

 will be convenient to deal with it separately. 



Sub-Clasg I, Archaeornithes. 



Only two specimens of Archceopteryx lithographica have hitherto 

 been found, both in the finely-grained lithographic limestone of 

 Solenhofen, Bavaria, belonging to the Jurassic period. The Bird 

 (Fig. 1067) was about the size of a Crow, and in the fossils not only 

 are the bones preserved, but also many of the feathers. 



The most striking feature in the organisation of this Bird is the 

 fact that the tail is composed of about 18 20 free caudal vertebrae 

 gradually tapering to the distal end as in a Lizard. The rectrices 

 are arranged in two rows, one on each side of the caudal vertebrae, 

 forming a long tail quite unlike that of any existing Bird. The 

 centra probably had biconcave faces. In addition to cervical and 

 thoracic ribs (which were apparently devoid of uncinates) there 

 were abdominal ribs, like those of Sphenodon and Crocodiles. 



The skull (Fig. 1068) is proportionately large, with rounded 

 brain-case and strong jaws, in each of which is a series of conical 

 teeth lodged in sockets. There is no trace of sternum in either 



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