VERTEBR.E AND RIBS 



45 



centra, while the more recent species, except those noted below, have procoelous 

 centra. The neural arches are connected by suture with the centra ; and cervicals 

 and anterior thoracics have a strong process (hypapophysis) on the ventral side 

 of each centrum. All of the cervicals (usually nine), the atlas included (fig. 27) 

 bear bicipital ribs. Between atlas and cranium is an incomplete osseous arch, 

 the proatlas (p. 27), preformed in cartilage, lying dorsal to the spinal cord. 

 The atlas consists of distinct neurapophyses and hypochordal bar, its centrum) 

 as in other Amniotes, forming the dens of the epistropheus. With this condi- 

 tion, the epistropheus cannot be procoelous. 



There are about ten thoracic vertebrae, the anterior two or three having 

 distinct di- and parapophyses, bearing bicipital ribs enclosing a well-marked 

 vertebarterial canal. Farther back the two transverse processes unite, although 

 the ribs show capitular and tubercular heads. The anterior ribs are 

 divided into vertebral, intermediate and sternal parts, the middle 

 being incompletely ossified. The two anterior ribs join the sternum 

 proper, while from five to seven of the others join its prolongation, 

 the xiphisternum. The lumbar vertebrae (about five) have well- 

 marked transverse processes, diapophysial in position. There are 

 two amphiplatyan sacral vertebrae, each with its sacral rib, con- 

 nected by suture with ilium and saqral. The caudal vertebras, , 

 the first biconvex, number thirty or more. The anterior have 

 strong transverse processes, and below are chevron bones, 

 apparently intervertebral in position, but in reality articulating 

 with the posterior ends of their centra. 



Rhyxchocephalia have delicate, usually amphicoele vertebrae, 

 but in a few fossils they are amphiplatyan. In front of the sixth, 

 intercentra usually occur between the centra, but are sometimes 

 lacking in the dorsal region. Sphenodon is the only hving reptile 

 with persistent intervertebral structures, these tending to reduce 

 the amphicoely of the centra. Thoracic and lumbar vertebrae are Yig. 51. 

 distinct. There are from two (5^//c«o(io«) to four sacrals. Some — Vertebra 

 of the fossils had enormously long spinous processes on the trunk process of 

 vertebrae, which in some general bore several cross bars of proble- Naosaums 

 matic function (fig. 51). Both cervical and thoracic ribs, the latter (Cope), 

 with a single head, are movably articulated in Sphenodon, in which 

 genus the ribs bear uncinate processes. The related Thalattosaurs have ribs 

 with a single broad head. 



Extinct Reptilian Orders. — The Theromorpha have amphicoelous vertebrae, 

 and the intercentra are rudimentary and confined to the cervical and anterior 

 dorsal regions. The cervicals often bear short bicipital ribs; those of the dorsal 

 region being longer and sometimes bicipital, sometimes with a single head. They 

 are sometimes articulated between two vertebrae and sometimes to the neural 

 arches. The sacral vertebrae are from two to four in number. 



The vertebrae of the Sauropterygia are usually amphiplatyan, but some- 

 times amphicoelous. The regions of the column vary in different genera, the 

 cervicals ranging from 16 to 72, the dorsals from 20 to 30; there are from two 



