Prof. Huxley on the Animals between Birds and Reptiles. 69 
3. A considerable number of caudal and lumbar, or dorsal, ver- 
tebree unite together with the proper sacral vertebre of a bird to 
form its “sacrum.” Jn reptiles the same region of the spine is con- 
stituted by the one or two sacral vertebra. 
4. In Birds the haunch-bone (ilium) 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. 
Tn 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 ankylosed 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 /tatite, or struthious birds, comprising 
the Ostrich, Rhea, Emu, Cassowary, Apteryx, and the but recently 
extinct (if they be really extinct) birds of New Zealand, Dinornis 
&ce., 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, andthe 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 
shghtly 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. 
