GENERAL STRUCTURE. 1219 



impression of a tail among some of these tracks renders it, however, 

 not altogether improbable that a few of them may be due to Birds. 



Apart from the foregoing extremely doubtful evidence, the earliest 

 undoubted occurrence of Birds is in the Upper Jurassic, where we 

 find Archceopteryx in the Kimeridgian of Bavaria. Laopteryx, from 

 the Jurassic of North America, which was described as a Bird, is 

 more probably a Reptile. Of the extremely generalised nature of 

 the former mention is made in the next chapter. In the Cretaceous 

 system of North America we find not only the remarkable toothed 

 Birds, which were already differentiated into the Ratite and Carinate 

 modifications ; but there were apparently others more nearly allied 

 to existing types. Bird-remains also occur rarely, and in a very 

 fragmentary state, in the Upper Cretaceous of Europe. With the 

 Eocene we enter upon an Avian fauna of a decidedly modern type ; 

 and at the period of the Lower Miocene the greater number of 

 existing suborders were well represented. 



We are at present to a great extent in the dark as to the manner 

 in which Birds branched off from the primitive Sauropsidan stock ; 

 but it is pretty evident that the Dinosauria are those Reptiles most 

 nearly related to Birds, and that the Ornithosauria are totally out of 

 the direct ancestral line, — the curious resemblance which they pre- 

 sent to Carinate Birds being apparently solely due to their somewhat 

 similar mode of life. 



In respect to the mode of origin of the Ratite and Carinate modifica- 

 tions of Bird -structure, we may quote from an admirable article by Pro- 

 fessor A. Newton, who observes that — " First of all we find that while 

 Birds still possessed the teeth they had inherited from their Reptilian 

 ancestors, two remarkable and very distinct types of the class had already 

 made their appearance, and we must note that these two types are those 

 which persist at the present day, and even now divide the class into the 

 Ratitse and Carinatae. Furthermore, while the Ratite type (Hesperornis) 

 presents the kind of teeth, arrayed in grooves, which indicate (in Rep- 

 tiles at least) a low morphological rank, the Carinate type (Ichthyornis) 

 is furnished with teeth set in sockets and showing a higher development. 

 On the other hand, this early Carinate type has vertebrae, whose com- 

 paratively simple biconcave form is equally evidence of a rank unques- 

 tionably low ; but the saddle-shaped vertebrae of the contemporary Ratite 

 type as surely testify to a more exalted position. Reference has been 

 already made to this complicated if not contradictory state of things ; 

 the true explanation of which seems to be out of reach at present. It 

 has been for some time a question whether the Ratite is a degraded type 

 descended from the Carinate, or the Carinate a superior development of 

 the Ratite type." The Professor, after noticing that many Zoologists 

 have adopted the former view, proceeds to observe that, before the ques- 

 tion can be answered, a reply must be given to the following question — 

 " Was the first animal which any one could properly call a ' Bird,' as dis- 

 tinguished from a ' Reptile,' possessed of a keeled sternum or not ? Now 

 Birds would seem to have been differentiated from Reptiles while the 

 latter had biconcave vertebrae, and teeth whose mode of attachment to 



