44 COMPARATIVE ANATOMY. 



nection with Birds. At the present day it cannot appear doubtful 

 to any morphologist that the latter are descended from Reptilian 

 ancestors. 



Birds. In Archseopteryx, found in the Solenhofen slates of 

 the Bavarian Jurassic, and already mentioned on p. 22, many of the 

 special peculiarities of Reptiles and Birds are united. The hinder 

 extremities are distinctly Reptilian, as is also the tail, which, like 

 that of a Lizard, is composed of numerous elongated free vertebrae. 

 A covering of true feathers, on the other hand, characterises it as 

 a Bird : the biserial arrangement of the tail-feathers is seen in 

 Fig. 33. 



FIG. 33. TAIL OF Archceoptcryx. 



The vertebral column of Birds corresponds with that of Reptiles 

 not only in its phylogenetic relations, but also ontogenetically. In 

 both groups the notochord eventually disappears entirely, and the 

 whole skeleton becomes strongly ossified. 1 The pelvis of Bird- 

 embryos, like that of existing adult Reptiles, is attached to the 

 vertebral column by two vertebrae only ; during further develop- 

 ment, however, a number of other vertebras (thoracic, lumbar, and 

 caudal) become fused with the sacrum (Fig. 34). 



A further difference between the vertebral column of Reptiles 

 and Birds is seen in the character of the caudal region in the latter 

 group, which always remains apparently rudimentary. In this 

 peculiarity existing Birds stand in sharp contrast to their Jurassic 

 ancestors (see above). 



It must, however, be well understood that the pygostyle of 

 Birds may be made up of six or more fused caudal vertebrae, and in 



1 Ichthyornis (from the American Cretaceous), as well as Archaeopteryx, pos- 

 sessed biconcave vertebrae. The same type of vertebra is to be met with in many 

 fossil (e.g. in the Enaliosauria), and in some existing Reptiles (Ascalabota and 

 Hatteria), as well as in most of the free caudal vertebras in existing Birds. 



