ME. ST. GEOEGE MIYAET ON THE SKELETON OE THE PEIMATES. 
334 
when the bones are naturally united, as when the tibia is vertical, then the long axis 
of the tuberosity of the os calcis inclines from below upwards and peronead, the outer 
surface of that bone tending towards the ground. 
Now, with this bending downwards and inwards of the outer part of the os calcis, 
a concomitant upward development of the peroneal side of the astragalus often takes 
place, causing the surface for the outer malleolus to form an acute angle with the 
upper part of the astragalus. This is the case in Troglodytes, Simia, the lower Simiidee, 
Cebus, and most of the inferior Cebidee, and in Hapale and Tarsius. This angle, how- 
ever, which is almost a right angle in Man, is nearly so in Hylobates and Pithecia, 
while in Ateles and Lagothrix it is obtuse, as also in the Lemuridse, especially the Nyc- 
ticebinse, where it is so much so that the peroneal surface becomes not far from hori- 
zontal. 
The angle formed with the top of the astragalus by the surface for the tibial mal- 
leolus is generally more or less obtuse, and most so in the Gorilla *, where it is 
almost on one plane with the upper surface (Plate XIII. fig. 7). In Man this is almost 
a right angle, and nearly so in Indris. 
The peroneal surface generally looks more or less backwards, but not so in Man and 
Ateles, and scarcely so in Hylobates, Lagothrix, Tarsius, and Cheiromys. 
The tibial malleolar surface is not generally so much smaller than the peroneal 
one, as in Man and the Gorilla ; but, on the other hand, in the Nycticebinse the pre- 
dominance of the outer one is yet greater. In Ateles the equality of the two surfaces 
is remarkable. 
When the bone is altogether detached and placed on a horizontal surface, the 
peroneal border of the upper surface, in Man and Ateles, is slightly below the tibial 
one, and this is still more the case in the Nycticebinee, and sometimes in Lemur. 
Iu the Chimpanzee and Hylobates the peroneal border, when the bone is so placed, 
is, slightly higher than the tibial one, and very much so in the Gorilla and the lower 
Simiidas and Cebidse. 
The head of the astragalus is sometimes much compressed ; this is the case in Ateles, 
but the compression is at its maximum in Loris. 
The groove for the tendon of the flexor longus hallucis is sometimes marked off by a 
sharp process from that for the flexor longus digitorum. It is more or less so in Ateles, 
Lagothrix, Lemur, and Galago, but most so in the Nycticebinse, where in Loris the 
tendon of the flexor hallucis is made to pass through almost a bony foramen by the 
large development of this process and the simultaneous intwisting of the tuberosity of the 
os calcis. 
Of the inferior articular surfaces the anterior one is relatively smallest (compared 
with the posterior one) in Man and the Gorilla. It is rather larger in the Chimpanzee 
and Plylobates, still more so in the lower Simiidsc, and largest, relatively, in the Nycti- 
cebinge, especially in Loris. 
* Noticed by .Professor Owes (Comp. Anat. of Yertebrates, vol. ii. p. 550). 
3 b 2 
