1. basi-occipital. 2. exoccipital. 3. supra-occipital. 4. basisphenoid. 5. alisphenoid. 6. parietal. 7. presphenoid. 8. orbitosphenoid. 9. frontal. 10. periotic, immediately below which is the tympanic. 11. lachrymal. 12. ethmo-turbinal. 13. maxillo-turbinal. 14. nasal. 15. mesethmoid. 16. vomer. 17. pterygoid. 18. palatine. 19. maxillae. 20. premaxillae. 21. squamosal. 22. mandible. 23. tympano-hyal. 24. stylo-hyal. 25. epi-hyal. 26. basi-hyal. Between this and the epi-hyal is the cerato-hyal. 27. thyro-hyal. 28. jugal. Nerve exits are indicated by Roman numerals.] The =occipital segment= is the most posterior of the three, and consists of four cartilage bones, which in the adult are commonly completely fused together. They surround the great =foramen magnum= (fig. 75, 2) through which the brain and spinal cord communicate. Forming the lower margin of the foramen magnum is a large flat unpaired bone, the =basi-occipital= (fig. 75, 5). Above this on each side are the =exoccipitals=, whose sides are drawn out into a pair of downwardly-directed =paroccipital processes=, which are applied to the tympanic bullae[141]. The inner side of each exoccipital is converted into the large rounded =occipital condyle= (fig. 72, 13) by which the skull articulates with the atlas vertebra. The dorsal boundary of the foramen magnum is formed by a large unpaired flat bone, the =supra-occipital= (figs. 72 and 75, 1), which is continuous with a small bone, the _interparietal_, prolonged forwards between the parietal bones of the next segment. In old animals the interparietal forms the hind part of a prominent ridge running along the mid-dorsal surface of the skull and called the =sagittal crest=, while the junction line of the occipital and parietal segments forms a prominent =occipital crest=. The plane in which the bones of the occipital segment lie is called the occipital plane; the angle that it makes with the basicranial axis varies much in different mammals.