240 
POPULAR SCIENCE REYIEW. 
haunch-bone is not greatly 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 ilium. The ischia always unite in 
the middle ventral line of the body (Plate XXVII. fig. 3). 
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 Eeptiles 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. 
(Plate XXVII. fig. 6). This is not the case in Eeptiles (Plate 
XXVII. fig. 5). 
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 tarsometatarsus 
(Plate XXVII. fig. 6). Eeptiles with completely developed 
hind-limbs have at fewest four toes, the metatarsals of which 
are all complete and distinct from one another (Plate XXVII. 
Although all existing Birds differ thus definitely from ex- 
isting Eeptiles, one comparatively small section comes 'nearer 
Eeptiles than the others. These are the Ratitce, or Struthious 
birds, comprising the Ostrich, Ehea, Emeu, Cassowary, Ap- 
teryx, and the but recently extinct (if they be really extinct) 
birds of New Zealand, the Dinornithidce, which attained 
gigantic dimensions. All these birds are remarkable for the 
small size of their wings, the absence of a crest or keel upon the 
breastbone, 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 approxi- 
mation to the reptilian type is but small, and the gap between 
Eeptiles and Birds is but very slightly narrowed by their ex- 
istence. 
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 Eeptiles more bird-like than living reptiles? 
And I shall endeavour to show that both these questions^must 
be answered in the affirmative. 
* It is almost always free — the Frigate bird presenting the only example 
of its ankylosis with the rest with which I am acquainted. 
