PLACENTAL MAMMALS 



576 ^LAL-tlN 1AL IVJLAiVllVlAl^S XXI. J- 



in various ways for locomotion, often by raising the heel off the ground, 

 so that the animals came to walk on the digits instead of the sole of 

 the foot (Fig. 354) and the number of toes became reduced. (3) The 

 number of teeth became reduced and their shape specialized, often 



Fig. 354 Fig. 355 



Fig. 354. Postures of the foot in mammals. A, plantigrade (bear); B, digitigrade (hyaena); 

 C, unguligrade (pig). (From Lull, after Pander and D 'Alton.) 



FlG. 355. Comparison of pairs of brains of archaic (left) and modern (right) mammals of 

 similar size. Olfactory lobes dotted, cerebral hemispheres oblique lines, cerebellum and 

 medulla, dashes. A, *Arctocyon and Canis; B, *P/ienacodus and Sus; C, *Coryphodon and 

 Rhinoceros; D, Uintatherium and Hippopotamus. (After Osborn, from Lull, Organic 

 Evolution, copyright 1917, 1929 by The Macmillan Co., and used with their permission.) 



by the addition of cusps and their fusion to make transverse or longi- 

 tudinal grinding ridges in herbivorous animals or cutting blades in 

 carnivores. (4) The brain of the earlier mammals resembled that of 

 reptiles (Fig. 355); later forms showed increasing development of the 

 non-olfactory part of the cortex, increase of the frontal lobes, and other 

 changes probably correlated with more complicated behaviour and 

 better memory. It has often been supposed that the brain becomes 

 relatively larger in later forms, but Edinger has shown by study of 



