HISTORY OF THE ARTIODACTYLA 



359 



with five digits or three, just as the tapirs 

 and nearly all the Eocene genera of perisso- 

 dactyls had four toes in the manus. Much 

 more important is the fact that the plane of 

 symmetry, which in the perissodactyls bisects 

 the third digit and is therefore said to be 

 mesaxonic, passes between the third and 

 fourth digit and is paraxonic. The third 

 and fourth digits always form an equal and 

 symmetrical pair and are the "irreducible 

 minimum," beyond which the number of toes 

 cannot be diminished. A single-toed artio- 

 dactyl would seem to be an anatomical im- 

 possibility ; at all events, such a monstrosity 

 was never known. Hence the term ' ' cloven " 



or ''divided" hoof, which seems to take the Fig. i86. — Leftfore- 

 ^. i i /. /. ,1 1 arm bones of the 

 solid hoof of the horse domestic Pig iSus 



as the norm ; but ''cloven scrofa). r., radius. 



,..,,,, , ., C^.ulna. o/., olecra- 



or divided, while ex- ^ou 

 pressing the appearance 

 of the foot with sufficient accuracy, is 

 erroneous, if taken to mean the splitting 

 of what was once continuous. 



Especially characteristic of the order 

 is the structure of the ankle, or "hock- 

 joint" of the hind limb. The ankle-bone, 

 or astragalus, has a double pulley, the 

 upper and lower ends being of quite similar 

 shape; its lower end is almost equally 

 divided between the cuboid and navic- 



L lunar. Py pyrami- j j^-^j^ ^^^ j^_^^^g COnCaVC tO reCCive 



dal. Fis., pisiform. Td., ' 



trapezoid. M., mag- it. This type of astragalus is altogether 



T/, second" m!°7/; peculiar to the artiodactyls, all of which 



third, Mc. in, fourth, posscss it j it is unlike that of any other 



fifth, meta- ^^^^^^j whatever and may be recognized 



Fig. 187 

 of Pig. 



— Left manus 

 »S., scaphoid. 



Mc. IV 

 carpals 



