26 



VERTEBRATE SKELETON 



(many Urodeles, fig. 280, Crocodilians, turtles, fig. 26, etc.). The 

 sacral vertebras may remain distinct or may fuse. Other vertebrae 

 (pseudosacrals) frequently enter into pelvic relations, but these, 

 lacking ribs, join the pelvis by their transverse processes. These 



Fig. 26. — Sacral vertebrae, sacral ribs and pelvis of Trionyx obliquely from below. 

 /, head of femur; //, ilium; /5, ischium; p, pubis; sr, sacral ribs; sv, sacral vertebrae. 



additions and the true sacrum make up a synsacrum which reaches 

 its extreme in birds (the oblique position of the body necessitating a 

 strong support), where the synsacrum may include twenty vertebrae. 



Fig. 27. — Three anterior vertebrae of Rhamphostoma (Crocodilian; Schimpkewitsch, 

 '21). a, atlas; c, capitular head of rib; d, diapophysis; ep, epistropheus; o, odontoid 

 process; p, parapophysis; pa, proatlas; r, cervical ribs, the arrow through the verte- 

 brarterial canal; /, tubercular head of rib. 



only two of which are true sacrals, the others being added from 

 caydal and lumbar regions. 



In Amphibia the presacral vertebrae, except the first (atlas), 

 bear ribs, the atlas here is regarded as forming a cervical region, the 



