378 PATAGONIAN EXPEDITIONS! PAL/E ONTO LOGY. 
narrow rim. A notch in the posterior margin divides this bounding ridge 
into two portions, the hypoconid and hypoconulid-entoconid. The latter 
cusps are not differentiated from each other. In worn teeth, the heel con- 
sists of two rounded eminences, separated by a shallow groove. In con- 
trast with the anterior molars, Mt has a much smaller heel, enclosing a 
small basin with a single prominent cusp on its posterior rim. An 
antero-external cingulum is present on the second, third and fourth tooth 
in the series. 
Milk Dentition. — Ameghino states (1894, p. 109) that in Cladosictis 
the canine, median and posterior premolars have deciduous predecessors. 
The mandible represented in fig. 6, PL LIX, retains the deciduous tooth 
replaced by the posterior premolar, the germ of which is visible beneath 
the anterior root of the former tooth. Two large alveoli precede the de- 
ciduous tooth, evidently for the roots of the median premolar. No tooth 
germs were found beneath them. The region occupied by the anterior 
premolar has been destroyed by fracture, and in repairing this break the 
canine has been too closely approximated to the molars. The root of the 
canine is hollow, indicating, not that it is a deciduous tooth, but that it 
was not yet fully erupted and had not ceased its growth at the time of the 
animal’s death. The tooth replaced by the posterior premolar differs from 
its permanent successor in the small size of the crown, which is greatly 
compressed laterally, carrying a central cusp preceded by an accessory 
basal cuspule. The narrow, ridge-like heel is subdivided by a shallow 
notch into two cuspules. The first and second molars are fully erupted 
and the third partly so. 
Skull (Plates LIV, fig. 1; LV, fig. 1; LVI ; LXI, fig. 1). — Perhaps 
the most striking peculiarity of this remarkable genus is the greatly elon- 
gated, narrow skull, altogether disproportionate to the size of the body. 
The facial portion is short and slender and the cranium elongated. The 
post-orbital constriction is proportionately greater than in the much 
smaller genus A mphiproviverra. The inclination of the upper border of 
the facial profile varies with the species. The brain (PL LVI, fig. 3) was 
less convoluted than in Thylacynus , and the brain case proportionately 
much smaller. The ascending premaxillary processes have approximately 
the same degree of development as in A mphiproviverra, and the nasals 
are similarly expanded posteriorly. Well defined post-orbital processes 
are present in C. lustratus. A short distance above these processes, 
