3. The =Orbits=. Each is bounded behind by the postorbital bar, externally by the jugal forming a continuation of the infratemporal arcade, in front by the lachrymal, and internally by the frontal and prefrontal. 4. The =Anterior nares=. These form an unpaired opening bounded by the premaxillae. _Posterior surface._ 5. The =Foramen magnum=. The exoccipitals form the chief part of its boundary, but part of the ventral boundary is formed by the basi-occipital. 6. The =Pterygoid fossae=. These form a pair of large cavities at the sides of the occipital region of the skull. The dorsal boundary is formed by the quadrate and quadratojugal, the ventral by the pterygoid, the internal chiefly by the quadrate, pterygoid, alisphenoid, and basisphenoid. The transpalatine forms a small part of the external boundary which is incomplete. _Ventral surface._ 7. The =Posterior nares=. These form a median unpaired opening (fig. 43, A, 5) bounded by the pterygoids. 8. The =Posterior palatine vacuities=. Each is bounded by the maxillae in front, the maxillae and transpalatine externally, the transpalatine and pterygoid behind, and the palatine on the inner side (fig. 43, A, 7). 9. The =Anterior palatine vacuity=. This is unpaired and is bounded by the premaxillae (fig. 43, A, 8). (_b_) THE LOWER JAW OR MANDIBLE. The mandible is a strong compact bony structure formed of two halves or =rami=, which are suturally united at the symphysis in the middle line in front. Each ramus is formed of six separate bones.