130 
EXTINCT MONSTERS 
Claosaurus (p. 169), and the Ignanodon (p. 164), reveal it consider- 
ably compressed vertically. In those forms it must have been a 
cumbrous impediment in the way of walking on land ; but in others, 
such as Anchisaurus (p. 142), the tail was rounder and more lizard- 
like, and doubtless these carnivorous forms were less hampered in 
their movements on land. Concerning the smallness of the fore 
limbs. Professor Owen suggests that the forward dash of the 
creature through the water in pursuit of fishy prey would be 
facilitated by this reduction in size of the fore limbs, which 
would take no part in movement, unless in the way of steering. 
Fig. 41. — Eggs of reptiles from the Great Oolite. 
On land they might be of some use in scratching out the nest for 
the eggs, if they did lay eggs, as well as in helping to support j 
the body. 
When any tribe of plants or animals becomes very flourishing, ; 
and spreads over the face of the earth, occupying regions far j 
apart from one another, where the geographical and other con- , 
ditions, such as climate, are unlike, its members will inevitably i 
develop considerable differences among themselves. 
During the great Mesozoic period. Dinosaurs spread over a i 
large part of the world ; they became very numerous and 
