342 



PRINCIPLES OF ANIMAL BIOLOGY 



forms was merely a process projecting down from above. The body had 

 increased to a height of 3 or 4 feet. 



Phohippus (Phocene) was not appreciably larger than the preceding 

 member of the series but the two lateral toes had disappeared (Fig. 290), 

 except as long splint bones. Pliohippus was thus the first one-toed 

 horse. The teeth were moderately long-crowned and possessed grinding 

 surfaces. The body stood about 48 inches high. 



The fossil horses of Pleistocene time were so nearly like the living 

 forms as to be included with the latter in the &ame genus (Equus). The 

 recent animals are 60 inches or more in height and weigh many hundreds 

 of pounds. Each foot has but one toe. Two lateral toes are evidenced 



Fig. 292. — Skull of Merychippus, about ^{q natural size. {From model prepared by 



Ward's Natural Science Establishment.) 



by splint bones, and in rare cases a reversionary horse is born with exter- 

 nally visible digits articulated with one of these splints on each forefoot. 

 The teeth are long and columnar and grow continuously during early 

 and middle life, during which time the wear at the upper surface approxi- 

 mately equals the growth. The grinding surface is worn fiat, except 

 that the enamel resists the abrasion more successfully than do the dentine 

 and cement, so that the enamel forms sharp cutting ridges. The posi- 

 tion of these ridges changes somewhat as the tooth is worn to different 

 levels and the pattern of the upper surface is indicative, in a general 

 way, of the age of the animal. Later in life growth of the teeth prac- 

 tically ceases, and then the teeth may become quite short. The face is 

 relatively longer than in the ancestral forms, since the eye is set well 

 back of the teeth and the brain case has not been relatively enlai-god. 



