Page:The Osteology of the Reptiles.pdf/109

Rh 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 zygapophysial articulations about their inner ends, known as zygosphenes (Fig. 73 ) and zygantra (Fig. 73 ).

The zygosphene is a wedge-shaped process at the anterior end of the arch, above and between the zygapophyses, which fits into a corresponding cavity, the zygantrum, at the posterior end of the next preceding vertebra. Zygosphenes and zygantra strengthen the articulations, though restricting vertical 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 hyposphene (Fig. 74) is below and between the inner ends of the postzygapophyses, 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 dinosaurs, where they were first recognized and described.

The later pterodactyls have another pair of articulating processes, called exapophyses (Fig. 73 ), at each end of the cervical vertebrae on the ventral side, their articulating surfaces facing in opposite directions to those of the zygapophyses above them. They strengthen the articulations, but limit torsion, and are substitutes