436 THE ANTIQUITY OF MAN 
blasts — carry out the transformation. It will be seen 
that the opening for the dental nerve has also to be 
carried backwards. For some reason which we under- 
stand only imperfectly at present, the migration of the 
dental opening is retarded in anthropoid jaws, but the 
mylo-hyoid groove is not. Hence in adult anthropoids 
the dental opening and the mylo-hyoid groove usually 
become separated as in the Piltdown mandible. 
Here there is a simian feature, but one on which we 
must not lay too much stress. It is a remarkable circum- 
stance that in very young anthropoids, especially in the 
gorilla (see fig. i6o), the human form of mylo-hyoid 
groove is present. In the most primitive forms of 
anthropoid apes — the gibbons — both the human and 
anthropoid arrangements of this groove are found. In 
monkeys the human form is the rule. In this character 
modern man seems to have relapsed to a more primitive 
condition. It will also be noted (fig. i6o) that the 
ridge for the mylo-hyoid muscle is more clearly indicated 
in young than in adult anthropoids. 
In a former chapter (p. 322) attention has been drawn 
to the most outstanding of all the characters of the Pilt- 
down mandible — the shelf or ledge of bone which unites 
the right and left halves in the region of the symphysis 
or chin (fig. 161). That feature has never before been 
seen in a human lower jaw : it is a characteristic of the 
anthropoid mandible. The mylo-hyoid ridge ends near 
the lateral borders of this simian shelf. 
We now come to a feature which is rather peculiar to the 
Piltdown mandible. On the inner side of the jaw, above 
the " simian " shelf and above the anterior indication of 
the myloid ridge, is a wide, shallow fossa (see fig. 161). 
I do not think there can be any doubt as to the nature of 
this fossa ; it is the impress of the salivary gland — the 
sublingual — which lies in the front part of the floor of 
the mouth above the mylo-hyoid muscle. In modern 
human jaws the impression of the sublingual gland is 
usually well marked, but variable in size (fig. 160). I 
have not seen any modern mandible in which the im- 
