SOME NOTES ON THE EARLY HOMlNIDiE 207 
The crucial differences between the human and simian 
jaw are found in the region of the anterior extremity — that 
region where the two halves are welded together to form the 
so-called symphysis menti. 
The chin is a modern human acquirement, and is absent 
in the apes. 
In the ape the contour of the jaw at its anterior extremity 
forms a sudden back-sweeping curve. The Heidelberg jaw 
Matter Jaw = thick line ; Australian 
Native = thin line ; Chimpanzee 
= BROKEN LINE. 
Vertical mesial section 
THROUGH THE SYMPHYSIS 
OF MAUER JAW (THICK 
LINE) ; AUSTRALIAN 
(THIN LINE) ; AND A 
CHIMPANZEE (BROKEN 
LINE). 
possess no chin, the contour here being exactly what one 
finds in the apes. This is well seen in Eig. 6. 
The internal surface of the region of the symphysis in the 
present races of man slopes sharply downwards from the 
posterior surface of the incisors, and as a rule does not show 
any differentiation into regions. In the higher apes the 
slope is very gentle, especially in its upper portion. With 
regard to this peculiarity the Heidelberg jaw holds an inter- 
mediate position. If the internal surface of the symphysis 
menti of a modern man is examined, it will be noted that in 
its lower one-third it shows what are called mental spines : 
the lower being a slight median ridge or roughness to which 
are attached the genio-hyoid muscles, the upper forming a 
