between the cervicals and the 2 sacrals bear ribs. Since the 

 thoracic and lumbar series are not distinct, these vertebrae 

 collectively are called dorsals. About 6 dorsals have ribs con- 

 nected with the sternum. Behind these complete ribs, the 

 ribs shorten in a graduated series until the last presacral (the 

 24th or 25th vertebra), which has only a nubbin, is reached. 

 The complete ribs, and those behind them, are distinctly 

 single-headed, and lack an uncinate process. All of the 

 dorsal (thoraco-lumbar) vertebrae have wide neural spines, 

 and distinct pre- and postzygapophyses. 



There are 2 sacrals supporting the pelvis, and nearly 50 

 caudals. From the fourth caudal to nearly the tip of the tail, 

 each vertebra has a Y-shaped hemal or intercentral arch 

 (caudal chevron). This arch articulates with the interverte- 

 bral gap. The caudal vertebrae have tall neural spines 

 tapering down and disappearing at about two-thirds of the 

 length of the tail. Neural arches with pre- and postzyga- 

 pophyses continue back nearly to the tip of the tail. The 

 anterior caudal vertebrae have long transverse processes 

 that double about one-third of the way back along the tail 

 and take part in the formation of a vertical breakage line 



in the middle of the body. About half way along the length 

 of the tail, the transverse processes disappear, but the 

 breakage lines occur to the tip of the tail. The vertebrae, 

 with the exception of the atlas and axis, are procoelous— 

 hollowed in front, but with a bulging articular surface be- 

 hind (Figure 6-10). 



Tuatara The neural arches of Sphenodon do not retain the 

 line of suture with the body as in the alligator, and an in- 

 tercentrum is present between all of the vertebrae. There 

 are 8 cervicals, unless one excludes the last, which bears a 

 large nb. This rib, however, is without a costal segment 

 attaching to the sternum. 



There is a small preatlas neural arch to either side in the 

 ligament joining the atlas, a ring of 3 pieces, to the skull 

 (Figures 6-5 D, 6-6). The atlas neural arches do not meet dor- 

 sally. The body of the axis has the expected composition; 

 its centrum is fused with its intercentrum, with the centrum 

 of the first cervical, and, perhaps, with the centrum of the 

 preatlas (Figure 6-9). The third cervical has a transverse 

 process, but no rib. There is a two-headed, connective tis- 



articulotion area 



on prezygapaphysis 



B 



intercentrum 



first haemal arch 

 (intercentrum or chevron 



breakage line 



procoracoid , 



sternum costal segment 



interclcvicle^ _ ^^^ „^„.v. 



gastrolia 

 Figure 6-6. Body skeleton of Sphenodon. A, vertebral column, girdles, and ribs with the gastrolia 

 rotated somewhat to produce a more ventral view; B, lateral view of two dorsal vertebrae with inter- 

 centra and ribs; C, anterior view of dorsal vertebra; D, lateral view of two anterior caudal vertebrae; 

 E, two middle caudal vertebrae. 



THE VERTEBRAL COLUMN • 147 



