372 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



fishes (Merospondyli^) and Batrachia (Rhachitomi). 

 Such was the origin of the segmentation in the primi- 

 tive sharks, Pleuracanthus, whose structure has been 

 pointed out by Sauvage, and Hybodus, whose charac- 

 ters have been demonstrated by Smith Woodward. 

 The segmented (or rhachitomous, as I have termed it,) 

 condition may be then regarded as the primitive one of 

 the osseous column in the Vertebrata. From the rha- 

 chitomous cohimn two divergent Hues have arisen as 

 already remarked (pp. 89,209). The inferior segment 

 has been retained in the fish-batrachian line, whence I 

 have termed their vertebrae ' ' intercentral, " while these 

 bodies have disappeared or become rudimental in the 

 higher Vertebrata. The pleurocentra (Figs. ii3-ii4a, 

 c, pi.') have, on the other hand, developed downwards, 

 and, meeting below, have formed the effective centrum 

 of the vertebra. Hence, in the Monocondylia and 

 Mammalia the vertebrae are "central." 



The Reptilia display a greater variety of vertebral 

 articulation than any of the classes of Vertebrata. 

 After the primitive biconcave (amphicoelous) type was 

 abandoned, the two principal types assumed are the 

 ball and socket (proccelous and opisthocoelous), 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 ex- 

 hibit 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 pro- 



1 Zittel, Uandbuch der Palceontologie , III., p. 138, 1887, where this charac- 

 ter'is first clearly pointed out in fishes. 



