shell-covered eggs; in some cases their young are brought forth 
alive. 
ORDERS: 
CROCODILIA —the crocodilians (alligators, caymans, crocodiles and 
gavials). These are four-limbed reptiles of medium or large size, 
four to thirty feet in length, with elongated jaws. There are now 
twenty-five species. 
RH YNCHOCEPHA LI A — the tuatara. A four-footed, lizard-like rep¬ 
tile. Only one species, found in New Zealand, is now extant. 
SQUAMATA —the lizards and snakes divided into two sub-orders: 
LACERTILIA — the lizards. Reptiles with the two halves of 
the lower jaw fused in an immovable bony suture. Most 
of the twenty-five hundred species have functional 
eyelids. 
OPHIDIA — the snakes. In snakes the two halves of the 
jaw are connected by an elastic ligament. They lack 
movable eyelids. About twenty-three hundred species 
are known. 
CHELONIA — the terrapins (found in fresh water), turtles (salt 
water) and tortoises (land). The limb girdles are enclosed in the 
ribs, which form a boxlike covering. There are about two hundred 
and fifty species. 
The following modern characteristics of the reptiles demonstrate the 
progress from aquatic to terrestrial life. The reptile skin is dry and cov¬ 
ered with horny scales or scale-like folds of skin, unlike that of the am¬ 
phibians which breathe principally through the skin and secrete a slimy 
substance to keep it moist. Breathing is performed almost entirely by 
the lungs. The mode of reproduction, too, is that of land rather than sea 
dwellers. 
Strictly speaking, reptiles are not “cold-blooded.” Their temperature 
is determined by the heat of their surroundings and the extent of their 
muscular activity. However, as they lack sufficient heat to hatch out their 
young, incubation, except in a few live-bearing species, is performed by 
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