GENERAL CHARACTERISTICS. 
3 
separated from Birds by the absence of feathers; the appendages of the outer 
layer of the skin being in the form either of overlapping horny scales, or of large 
shields uniting by their opposed edges. Moreover, all known Reptiles differ from 
Birds in having more than three digits in the fore-limb; while in no cases are 
the collar-bones fused into a furcula, as they are in all flying Birds. A further 
distinction is to be found in connection with the circulatory system, the blood of 
all existing Reptiles being cold, while the aorta, or great propelling blood-vessel of 
the heart is double, and 
crosses both branches (in¬ 
stead of only the left branch) 
of the windpipe. It will 
be obvious, however, that 
these two last characters 
cannot be verified in the 
case of extinct Reptiles, 
among which it is quite 
probable that there may 
have been some in which 
the blood was warm. A 
similar remark will apply 
to the absence among living 
Reptiles of those ramifica¬ 
tions of the bronchial tubes 
throughout the body, which 
form such a characteristic 
feature in the structure of 
Birds. As additional features 
in the skeleton, it may be 
noticed that Reptiles never 
have the terminal faces of 
the vertebrae saddle-shaped; 
while in those forms in which the number of toes in the hind-limb is reduced to three, 
the metatarsal bones do not unite to form a cannon-bone in conjunction with the 
lower row of bones belonging to the ankle-joint. Then, again, with the exception 
of one remarkable extinct group, Reptiles, as a rule, are characterised by the three 
bones of the pelvis remaining distinct from one another through life; whereas in all 
existing birds they are welded together. There are likewise differences in regard 
to the form and structure of the breast-bone and sacrum, into the consideration of 
which it will be unnecessary to enter in this work. 
Diversity of In marked contrast to the uniformity in appearance and structure 
characterising Birds, the various groups of Reptiles differ widely from 
one another, both as regards external form and internal structure. 
Externally, a lizard, a snake, and a tortoise present the most marked differences in 
general appearance among living members of the order; while among extinct types 
there were some which walked on their hind-limbs alone, after the manner of Birds, 
and others having their fore-limbs modified into wings and the digits connected 
THE BONES OF THE LEFT SIDE OF THE PELVIS OF AN EXTINCT 
DINOSAURIAN REPTILE (-jL nat. size). 
il, ilium ; p, pubis ; is, ischium.— After Marsh. 
Form and 
Structure. 
