The external surface of the basis cranii of Zinjanthropus is beautifully preserved (pls. 6 and 14). With the exception of a few parts of the left temporal bone and the apex of the right petrous pyramid, the entire basis is preserved, down to the finest anatomical detail, as far forward as the body and greater wings of the sphenoid bone. Furthermore, the parts which are missing or damaged on one side are intact on the opposite side.

The occipital bone

The planum nuchale

The character of the nuchal crest has been dealt with in the preceding chapter. Immediately below the nuchal crest, on either side of the external occipital protuberance and of the posterosuperior part of the external occipital crest, is a clear oval impression, about 27 by 16 mm., in the area between the superior and inferior nuchal lines. In modern man, the semispinalis capitis muscle is attached to a corresponding impression. Anterolateral to the last impression is another somewhat irregular impression, corresponding to the attachment of m. obliquus capitis superior. Posterolaterally, the lower lateral limb of the nuchal crest is rugged, as it flows towards the adjacent pars mastoidea and mastoid process: this part probably provided attachment to the splenius capitis muscle.

The inferior nuchal line is not very clearly defined. It runs below and in front of the impression for semispinalis capitis; then continues laterally for a short distance, turning sharply forwards opposite the impression for m. obliquus capitis superior, finally flowing over the lateral part of the exoccipital and terminating in the jugular process.