Sinclair: typotheria of the santa cruz beds. 
3 
2 . Interatherium. Dental formula -|, 1, |, with diastemata between the lateral incisor, canine 
and first premolar, varying with the species. D reduced, often wanting ; upper molars 
with deep internal inflection and prominent antero-external ridges ; Mg externally tri- 
lobate ; temporal bar of maxillary with strong descending process ; humerus without 
internal epicondylar foramen ; terminal phalanges laterally compressed hoofs, with or 
without clefts. 
(/. robustum , extensum, excavatum, Santa Cruz formation, Patagonia.) 
B. Family : HegetotheriidtE. Median incisor rootless ; third and fourth premolars molariform ; 
mastoid dilated inclosing a large hollow cavity ; malar large, excluding maxillary from orbit ; 
carotid canal and foramen lacerum posterius widely separated ; tibia and fibula firmly fused 
both proximally and distally ; pes approaching mesaxonic symmetry with digit III the 
longest, digit V greatly reduced and digits II and IV shorter than III, but robust; astraga- 
lar trochlea bilaterally asymmetrical ; navicular and calcaneum in articulation ; a small 
fibulo-calcaneal facet. 
1. Hegetotherium. Dental formula i, |, 1^, ^ and g vestigial ; canine vestigial ; upper 
molars internally convex, without inflection except in M-^- ; ectoloph smooth ; terminal 
phalanges greatly flattened transversely with conspicuous clefts. 
(//. mirabile, Santa Cruz formation, Patagonia). 
2. Pachyrukhos. Dental formula y L, -|, |-. All the upper molars internally convex ; ecto- 
loph smooth ; terminal phalanges hoof-like, without clefts in Santa Cruz species. 
[P. moyani, Santa Cruz, formation, Patagonia.) 
The Santa Cruz typotheres are animals of somewhat rodent-like appear- 
ance, varying in size from a cotton-tail rabbit to a cavy. A review of the 
more important skeletal characters of the group, although involving some 
repetition, may be of interest. 
i. The Skull. — The facial portion of the skull is slender and more or 
less excavated longitudinally, while the cranium is broad and well 
expanded. The orbits are central, circular in outline, quite prominent in 
Hegetotherium , Pachyrukhos and Interatherium and unenclosed pos- 
teriorly. The arches are robust in all except Pachyrukhos and moderately 
expanded. The premaxillae are short and heavy, with scarcely any ascend- 
ing process, the nasals are broad posteriorly, tapering forward to blunt 
points, the interorbital tract is plane and the sagittal and lambdoidal crests 
low. The most prominent feature of the back of the skull is the greatly 
distended mastoid tract, which may either be filled with cancellae or lodge 
a large cavity. In either case there is direct communication with the 
tympanic bulla and the dilatation appears to have functioned as a secondary 
resonator, perhaps associated with nocturnal habits. The palate is con- 
cave throughout, terminating posteriorly in a pair of stout processes. The 
mandible is heavy and deep without trace of suture in the firmly fused 
symphysis. 
