THE VERTEBRAE 



9r 



known as zygapophyses. The pair in front, the prezygapophyses {az), 

 always has the flat or concave articular surface directed upward, 

 that is, toward the dorsal side, or upward and inward ; while that of 

 the posterior pair, the postzygapophyses (pz), is turned downward, 

 that is, toward the ventral side, or downward and outward. The 

 zygapophyses may be obsolete or even absent in the posterior part 

 of the column of aquatic reptiles. 



The vertebrae of all snakes, some lizards, and some mosasaurs, 

 have additional articulations, or rather, extensions of the zygapo- 

 physial articulations about their inner ends, known as zygosphenes 

 (Fig. 73 D, f) and zygantra (Fig. 73 e). The 

 zygosphene is a wedge-shaped process at 

 the anterior end of the arch, above and be- 

 tween the zygapophyses, which fits into a 

 corresponding cavity, the zygantrum, at 

 the posterior end of the next preceding ver- 

 tebra. Zygosphenes and zygantra strength- 

 en the articulations, though restricting ver- 

 tical flexure. They occur, as is seen, only 

 in reptiles with a long, flexible vertebral 

 column/ and are absent in those mosasaurs 

 in which the column is less elongate and 

 flexuous. Zygosphenes are also known to 

 occur in certain aquatic Stegocephalia with 

 long, slender vertebral columns. 



In certain other reptiles this arrangement 

 is reversed, in that the wedge-shaped median process, called hypo- 

 sphene (Fig. 74) is below and between the inner ends of the post- 

 zygapophyses, fitting into a cavity, the hypantrum, at the front end 

 of the next succeeding vertebra. Hyposphenes and hypantra are 

 especially characteristic of certain cotylosaurs, placodonts, and dino- 

 saurs, where they were first recognized and described. 



The later pterodactyls have another pair of articulating processes, 

 called exapophyses (Fig. 73 g), at each end of the cervical verte- 

 brae on the ventral side, their articulating surfaces facing in oppo- 

 site directions to those of the zygapophyses above them. They 

 strengthen the articulations, but limit torsion, and are substitutes. 



Fig. 74. Dorsal vertebra: 

 Diadectes (Cotylosauria) from 

 behind, showing diapophyses, 

 postzygapophyses, and hy- 

 posphen*". 



[Exceptions to this rule occur in recent lizards. — Ed.] 



