6 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
Fig 
same in Protypotherium , but in Pachyrukhos eight lumbars are present. 
Five vertebrae are coossihed in the sacral complex, of which three are true 
sacrals in contact with the ilium, and two belong to the caudal series. The 
length of the tail seems to have varied. In Protypotherium and Intera- 
therium it is both long and heavy, while in Pachyrukhos there is reason to 
believe that it was quite short. 
4. Foot Structure. — Almost nothing has hitherto been known of the 
structure of the feet in the Santa Cruz typotheres, but definite information 
is now available for all the genera except Hegetotherium , in which the 
manus is still unknown, but from the close resemblance 
of Hegetotherium and Pachyrukhos it is probable that 
it was not unlike that of the latter, which, in turn, does 
not differ materially from the manus of Interatherium 
and Protypotherium (text fig. 3, A). In the Santa Cruz 
forms both manus and pes are tetradactyl, without the 
slightest trace of an opposable thumb or great toe. 
The carpus is strongly interlocking, without free cen- 
trale. Two types of hind feet are developed (text figs. 2, 
4, A) simulating the paraxonic and mesaxonic symmetry 
of the feet of the Artiodactyla and Perissodactyla re- 
spectively. These are probably to be correlated in the 
Typotheria with cursorial and saltatorial modes of pro- 
gression. Pachyrukhos was certainly a jumping animal, 
as shown by the greater length and strength of the hind 
limbs and inner digits of the pes. In fact, the struct- 
ure of both the fore and hind limbs in this animal 
closely resembles that of a hare. From the numerous 
structural similarities between Pachyrukhos and Hege- 
totherium it may be inferred that the latter was also 
saltatorial. Its broad, shallow astragalar trochlea is in 
contrast with the narrow, more deeply incised trochlea 
of the cursorial Protypotherium and Interatherium. 
Both of these genera have limbs of approximately 
equal length. The terminal phalanges in the Santa 
are hoof-like and in Hegetotherium have conspicuous 
/// 
Left hind foot of 
Protypotherium aus- 
tra/e, three fourths the 
natural size. ( No. 
9149 American Mu- 
seum of Natural His- 
tory collection.) 
Cruz typotheres 
median clefts. 
