THE VERTEBRATA 235 



size, the largest forms being chiefly found in the Pliocene 

 period, after which glacial conditions exterminated the 

 majority of them. The brain also increased greatly, 

 and limbs and teeth underwent various specializations in 

 accordance with the food and habitat. 



The most interesting developments are perhaps shown 

 by the Ungulata (hoofed mammals). In these the teeth 

 become high-crowned (hypsodont) to stand long wear, 

 and develop varying patterns which as they wear down 

 show a surface partly of hard enamel, and partly of soft 

 dentine, and therefore never wearing smooth. The limbs 

 become longer and vertical in position, lifting the body 

 high off the ground, while at the same time they lost the 

 power of rotation (useless to a running animal) by the 

 fusion of the two bones in the second limb-segment. In 

 one branch of the Ungulata (Perissodactyla, odd-toed) 

 the middle digit bears the weight of the body, and the 

 others gradually shorten and finally are nearly lost in the 

 modern horse (Equus), the ancestry of which has been 

 very fully worked out. In another branch ( Artiodactyla, 

 even-toed) the weight is borne by the third and fourth 

 digits (the " cloven " hoof), the first disappearing, as may 

 also the second and fifth : these include the cattle, deer, 

 and camels. In the Proboscidea, again, while the teeth 

 undergo enormous specialization, the limbs remain com- 

 paratively primitive, all five digits persisting, and fusion 

 of the bones of the second segment taking place in the 

 fore-limb only. Until recently the Proboscidea were a 

 cryptogenetic group, appearing suddenly in Europe in 

 the Miocene period, but their earlier ancestors have 



