3 2o COMPARATIVE ZOOLOGY OF CHORDATES 



extremity is expanded and surrounded by the nail which gives 

 rise to the hoof. The other digits have disappeared, leaving 

 only small vestiges of the metacarpals and metatarsals (of the 

 2nd and 4th digits) in the form of " splint-bones.". The 

 fossil ancestors of the horse show different stages in this 

 process of reduction of the number of digits, and lead back 

 to normal pentadactyl animals. These odd- toed Ungulates 

 are called Perissodactyls. Curiously enough, a parallel process 

 of reduction in number of digits, and of formation of hoofs 

 consisting of a single digit, went on in a group of animals (all 



°o o O O 



FlG. 164. — Convergence in the adaptation of limbs for swimming, in, A, 

 Ichthyosaurs ; B, Plesiosaurs ; C, birds (penguin) ; D, mammals 

 (dolphin) . 



now extinct) quite independently of the horses : the Thoatheria. 

 This is a very remarkable case of convergence in evolution. 



In the " cloven-hoofed " Ungulates or Artiodactyls, the 

 hoof is formed from the end joints of digits 3 and 4, as in cattle, 

 where the metacarpals and metatarsals of the two digits 

 fuse. 



Among mammals, limbs with a primitive type of structure 

 are those of the Primates, which preserve all the five digits. 

 In most Primates, the first digit (thumb or big toe) is capable 

 of touching any or all the remaining digits, i.e., is opposable. 

 This structure enables the limb to grasp objects firmly. Apes 



