REPRESENTATIVE LAMARCKIANS 259 



vertebra 'intercentral,' while these bodies have dis- 

 appeared or become rudimental in the higher Verte- 

 brata. The pleurocentra have, on the other hand, 

 developed downwards, and, meeting below, have 

 formed the effective centrum of the vertebra. . . . 

 "The Reptilia display a greater variety of vertebral 

 articulation than any of the classes of Vertebrata. 

 After the primitive biconcave (amphiccelous) type 

 was abandoned, the two principal types assumed are 

 the ball and socket {Proccelous and opisthoccelous) 

 and the plane (amphiplatyan) . In those families in 

 which the body is more or less in contact with the 

 ground, owing to the absence, shortness, or position of 

 the limbs (Lacertilia, Ophidia) , the vertebral bodies 

 exhibit the ball-and-socket articulation, while in types 

 with longer limbs which supported the body in pro- 

 gression, so that the latter never reached the ground 

 (Dinosauria) , the articulations are plane. The ball- 

 and-socket articulation may be inferred to have been 

 produced by vermiform movements which utilise 

 points of resistance on the earth as aids to progression, 

 while the plane articulation has probably resulted 

 from the persistence of the fixed relation which is ap- 

 propriate to a body which should be relieved by the 

 legs of all share in movements necessary to progres- 

 sion. That this position is correct is sustained by the 

 fact that the cervical vertebra? of various reptiles and 

 mammals which have plane dorsal vertebras have the 



