602 



HISTORICAL GEOLOGY 



A< 



history. The disadvantage of great size has been discussed (p. 550). 

 Mammalian Teeth. — The typical or ancestral number of teeth 

 is 44, a number which is seldom found in living forms, since some 

 have been developed at the expense of others and some have 

 been dropped. It is seldom that a larger number occurs. (The 

 porpoise has 246.) The primitive type of grind- 

 ing teeth (molars), from which the highly per- 

 fected teeth of the present carnivorous and 

 herbivorous mammals were derived, had a grind- 

 ing surface merely roughened by three sharp- 

 pointed cones arranged in the form of a triangle 

 (tritubercular). From a tooth of this simple 

 type has been developed the complicated and 

 efficient grinders of the higher herbivores (Fig. 

 543). Another notable change, in such races as 

 the horse and the elephant, has been the lengthen- 

 ing of the tooth, adapting it to the nutritious 

 grasses (p. 610) of the dry, sandy plains. 



Feet. — The primitive foot (p. 598) is five- 

 toed with the sole resting flat on the ground. 

 From such a foot, the extremely effective one of 

 the higher hoofed, herbivorous mammals was 

 developed. This was accomplished (1) by the 

 raising of the ankle and wrist joints which lifted 

 the first and fifth toes from the ground so that 

 these toes became useless, and degeneration set 

 in which eventually, as in the case of the horse, 

 and the modern horse causec [ a u except the middle toe to disappear. 

 (above). Ihe relative /NT , . . . r , .. r . . 



(2) In the primitive foot the joints of the wrist 



and ankle were loose, but they became more 

 efficient by the development of the " tongue and 

 groove " structure which very effectively pre- 

 vented lateral movement. The change, in general, has been from a 

 loose-jointed limb with "ball-and-socket" joints, to one with keeled 

 joints ; from walking with the sole of the foot flat on the ground 

 (plantigrade), to walking on the toes with the heel well elevated 

 above the ground (digitigrade) ; from a five-toed foot to one with 

 a smaller number of functional toes. It should not be forgotten, 

 however, that along with races that were changing in structure, 

 there lived others that have been little modified. 



Fig. 543. — Corre- 

 sponding grinding teeth 

 of Eohippus (below) 



size and efficiency are 

 shown. (After H. F 

 Osborn, Age of Mam- 

 mals.) 



