612 
REPTILIA. 
Some of the main contrasts between living Reptiles and 
Birds are summarised in the following table :— 
REPTILES. 
Birps. 
The exoskeleton consists of horny 
epidermal scales, sometimes augmented 
by bony dermal scutes. , 
The centra of the vertebrae are rarely 
like those of birds. 
When there is a sacrum, its vertebrze 
(usually two in number) have large ex- 
panded ribs with the ends of which the 
ilia articulate. 
The cartilaginous sternum may be- 
come bony, but is not replaced by 
membrane bones, unless perhaps in 
Pterodactyls. 
When there is an interclavicle or epi- 
sternum, it remains distinct from the 
clavicle and sternum. 
The hand has more than three digits, 
and at least the three radial digits are 
clawed. 
In living reptiles the ilia are prolonged 
farther behind than in front of the aceta- 
bulum ; the pubes slope downward and 
forward; there are usually pubic and 
ischiac symphyses. 
There are often five toes; the tarsals 
and the metatarsals remain distinct. 
At least two aortic arches persist ; 
only the Crocodilia have a structurally 
four-chambered heart; more or less 
mixed blood always goes to the pos- 
terior body. 
The body has approximately the tem- 
perature of the surrounding medium. 
The optic lobes lie on the upper 
surface of the brain. 
There is an outer covering of feathers, 
and though there may be a few scales, 
there are never scutes. 
‘The centra of the cervical vertebra 
| have usually a saddle-shaped terminal 
curvature. 
The two sacral vertebre have no 
expanded ribs, they fuse with others 
to form a long composite ‘‘synsacrum.” 
The cartilaginous sternum is replaced 
by membrane bone from several centres. 
When there is an interclavicle, it is 
confluent with the clavicles. 
The hand has not more than three 
digits, and at most two digits are 
clawed. The fore-limbs are modified 
as wings; some carpals fuse with the 
fused metacarpals. 
The ilia are greatly prolonged in front 
of the acetabulum, the inner wall of 
which is membranous. The pubes slope 
backwards, parallel with the ischia; 
only in Struthio is there a pubic 
symphysis, only in Rhea is there an 
ischiac one. 
There are not more than four toes; 
the proximal tarsals unite with the 
tibia, forming a tibio-tarsus; the first 
metatarsal if present is free, but the 
three others are fused to one another 
and to the distal tarsals, forming a 
tarso-metatarsus. 
There is but one aortic arch, to the 
right; the heart is four-chambered ; 
the blood sent to the body is purely 
arterial. 
The body temperature is high and 
almost constant. 
The optic lobes lie on the sides of the 
brain. 
The lungs have associated air-sacs. 
The sutures between the bones of the 
skull are usually obliterated at an early 
stage. 
The right ovary atrophies. 
