( 370 ) 
relation be tv 
basal index 
enumeration shows that 
to the acquisition of an 
es maj also be inferred that there is no'direct causal 
the position of the foramen magnum and the deve¬ 
lopment of the biped gait. Although it will presently appear that the 
lower than in the other Primates, the given 
oust be cautious in attributing this only 
gait. If this were the case, Siamanga 
would certainly not be at the top with the highest basal index but one. 
And as to the genus with the highest index value — Mycetes - 
it is not unlikely that here we have a secondary occipital shifting 
of the Foramen magnum as the result of the extraordinary develop¬ 
ment of the hyoid apparatus. The place, occupied by Mycetes, is an 
. exceptional one on account of peculiar developmental phenomena, 
faking this into account, Siamanga, the animal which is perhaps 
more skilled in biped walking than any other Primate, mentioned 
m this paper, stands at the top as to the value of its basal indej 
and approaches most closely the half-apes of which e.g. Lemur albi- 
rons had an index 87 and Propithecus diadema 80. But one must 
be careful not to place Siamanga on this ground only lower in the 
system than the Primates with a smaller basal index and especially 
not to look upon this phenomenon itself in Siamanga as the preserva¬ 
tion of an original condition. Of this I am not convinced. For, as 
wi be shown in a following paper, the position of the foramen 
magnum is largely determined by the position of the facial skull 
with respect to the cerebral skull.' And in the Primates an evolu¬ 
tional line in this respect may certainly be recognised, being in short 
that the facial skull, first situated before the cerebral skull, is dis- 
p ace under it, after which it is shortened. Now this occipital dis- 
p acement of the facial skull has a great influence on the position 
of the foramen magnum. 
At the bottom of the series stands with the lowest index Chryso- 
thrix as the representative of the platyrhine monkeys and Cercopi- 
t ecus of the catarrhine. For these latter an examination of more 
individuals, specially of the same species, seems desirable to me, in 
order to settle whether indeed in the genus Cercopithecus the foramen 
magnum is in general placed so much to the front. The Anthropoids 
o not, as far as their basal index is concerned, occupy a conspi¬ 
cuously favoured position among the apes and certainly do not form 
a irect link between the lower apes and man. 
. rh ? basal index of man is given in the following list, in which 
e different groups mentioned above, are kept separate. For the 
sake of completeness I also mention the cephalic index. 
