164 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 
there was no definite division of the plate into dorsal and ventral pro- 
cesses. The acetabulum is large, of nearly circular shape and has prom- 
inently projecting borders and is deeply invaded by the very large liga- 
mentous sulcus, which is out of all proportion to the size of the pit for 
the round ligament on the head of the femur. 
The ischium is proportionately elongate and, for most of its length, is 
slender and shallow dorso-ventrally, but quite thick transversely ; pos- 
teriorly, it expands into a thin and laterally compressed area, which has 
no definite tuberosity, but merely a low, rugose area. The pubis is quite 
broad, depressed and flattened and the symphysis is long. The large 
obturator foramen forms an almost regular oval, with the long axis 
directed antero-posteriorly. 
In Toxodon the pelvis is remarkably different from that of Nesodon and 
has acquired considerable resemblance to the pelvis of the elephants. The 
iliac plate is immensely expanded and very strongly everted and the crista 
has become very massive and rugose, especially near the antero-dorsal and 
postero-ventral angles. The large, circular acetabulum has a less promi- 
nent border and is not invaded by a sulcus for the round ligament ; there 
is merely a notch of the posterior border. All the post-acetabular region 
of the pelvis has been much reduced in length ; the ischium is relatively 
much shorter and thicker than in Nesodon and the tuberosity is a distinct 
ridge, though not at all prominent. The pubis is very heavy and the sym- 
physis is short, — actually but little longer than in Nesodon and therefore 
very much shorter proportionately. 
The very striking changes which the pelvis of Toxodon has undergone 
are all to be correlated with the great increase in stature and body- 
weight and in the mass and bulk of the viscera. Similar changes in the 
form of the pelvis, accompanying great increase in the size and bulk of the 
body, may be observed in several other ungulate phyla. Besides the 
Proboscidea, they are to be found in the Amblypoda and in the perisso- 
dactyl family of the Titanotheriidae and, in a less marked degree, in the 
larger rhinoceroses. In several of these groups the changes in the pelvis 
may be followed in successive stages, where they are plainly correlated with 
increasing body-weight and, as plainly, they are seen to be independently 
acquired in each phylum. To this list should now be added the Toxo- 
donta, the later and larger representatives of which have taken on many 
skeletal resemblances to the elephants, but, as the phylogenetic history 
