KINETOGENESIS. 303 



important modifications in these articulations are to be 

 seen in Vertebrata, the Reptiha presenting the great- 

 est variety, excepting in the zygapophyses, which are 

 tolerably uniform in that class. In the Mammalia, 

 modifications of the central articulations are not more 

 striking than are those of the zygapophyses. 



The forms of central articulation are four, viz. : the 

 amphicoelous, the ball-and-socket, the plane, and the 

 saddle-shaped. The first type is onl}' seen in a very 

 imperfect degree in Mammalia and in but very few 

 vertebrae, where it is indeed but a modification of the 

 plane. The ball-and-socket is chiefly found in the 

 neck of the long-necked Mammalia, as the higher 

 Diplarthra, and to a less degree in their lumbar re- 

 gions, while the dorsal vertebrae present an approach 

 to the same type in the same groups. The saddle- 

 shaped centrum is only found in Mammalia in the 

 necks of certain genera of monkeys. The majority of 

 Mammalia present the plane articulation of all the ver- 

 tebral centra. 



In Mammalia in which movement of the vertebrae 

 on each other has become impossible, the centra co- 

 ossify, as for instance in the sacrum. In this region the 

 number of vertebrae coosified is directly as the length 

 of the iliac bone, which supports and holds them im- 

 movable. Such is their condition throughout the 

 dorsal region in the extinct Edentata of the family 

 Glyptodontidae, where the carapace is, as in the tor- 

 toises, inflexible, and which therefore limits the possi- 

 bility of motion of the vertebral column. Another 

 illustration is seen in the necks of the balaenid Ceta- 

 cea, and to some degree in the Delphinidae and Physe- 

 teridae. The lack of present mobility of this part of 

 the column is due to its extreme abbreviation, a char- 



