Prof. Huxley on the Animals hetioeen Birds and Rejytiles. 69 



3. A considerable number of caudal and lumbar, or dorsal, ver- 

 tebra) unite together with the proper sacral vertebrae of a bird to 

 form its " sacrum." In reptiles the same region of the spine is con- 

 stituted by the one or two sacral vertebrae. 



4. In Birds the haunch-bone (ihum) extends far in front of, as 

 well as behind, the acetabulum ; the ischia and pubes are directed 

 backwards, almost parallel with it and with one another ; the ischia 

 do not unite in the ventral middle line of the body. 



In reptiles, on the contrary, the haunch-bone is not produced in 

 front of the acetabulum ; and the axes of the ischia and pubes diverge 

 and lie more or less at right angles to that of the illium. The ischia 

 always unite in the middle ventral line of the body. 



5. In all birds the axis of the thigh-bone lies nearly parallel with 

 the median plane of the body (as in ordinary Mammalia) in the 

 natural position of the leg. In reptiles it stands out at a more or 

 less open angle with the median plane. 



6. In birds, one half of the tarsus is inseparably united with the 

 tibia, the other half with the metatarsal bone of the foot. This is 

 not the case in reptiles. 



7. Birds never have more than four toes, the fifth being always 

 absent. The metatarsal of the hallux, or great toe, is always short 

 and incomplete above. The other metatarsals are ankjlosed together, 

 and unite with one half of the tarsus, so as to form a single bone, 

 which is called the tarso-metatarsus. 



Reptiles with completely developed hind limbs have at fewest 

 four toes, the metatarsals of which are all complete and distinct from 

 one another. 



Although all existing birds differ thus definitely from existing 

 reptiles, one comparatively small section comes nearer reptiles than 

 the others. These are the liatitce, or stnithious birds, comprising 

 the Ostrich, Rhea, Emu, Cassowary, Apteryx, and the but recently 

 extinct (if they be really extinct) birds of IN^ew Zealand, Dinornis 

 &c., which attained gigantic dimensions. All these birds are remark- 

 able for the small size of their wings, the absence of a crest or keel 

 upon the breast-bone, and of a complete furcula ; in many cases, for 

 the late union of the bones of the pinion, the foot, and the skull. In 

 this last character, in the form of the sternum, of the shoulder-girdle, 

 and in some peculiarities of the skull, these birds are more reptilian 

 than the rest ; but the total amount of approximation to the reptilian 

 type is but small, and the gap. between reptiles and birds is but very 

 slightly narrowed by their existence. 



How far can this gap be filled up by a reference to the records of 

 the life of past ages ? 



This question resolves itself into two : — 



1. Are any fossil birds more reptilian than any of those now 

 living? 



2. Are any fossil reptiles more bird-like than living reptiles ? 

 And I shall endeavour to show that both these questions must be 

 answered in the affirmative. 



