124 
PATAGONIAN EXPEDITIONS PALEONTOLOGY. 
B. Lower Jaw (Pis. XIII, XVIII, fig. 4). — As in the case of the upper 
incisors, those of the mandible greatly change their shape and appearance 
with age, though the changes are hardly so striking. All of the lower 
incisors are more or less strongly procumbent, but there is a considerable 
variation in the degree of procumbency among the individuals of the same 
species, though it never approximates the condition seen in Toxodon. 
When first erupted, i T is a broad, much depressed, scalpriform tooth, con- 
tracting gradually to the base and of trihedral cross-section, with thickened 
medial border, which forms the base of a narrow triangle, and thinning 
toward the external border, the apex of the triangle. When the root has 
been formed and growth ceases, abrasion causes the tooth to become nar- 
rower and thicker and assume a more definitely trihedral form. 
The second incisor is similar, but somewhat broader, and under- 
goes much the same changes of form. The third, on the other hand, 
grows throughout life from a persistent pulp and in the adult is a 
large and characteristic tusk, which bites behind the upper tusk and 
abrades its posterior surface, while i-3 itself is obliquely truncated by the 
wear. In this stage of development the tooth points obliquely upward 
and forward and its principal diameter is antero-posterior ; in shape, it is 
trihedral, with the base of the triangle formed by the antero-inferior border 
and the apex by the postero-superior. I3 is not erupted until after iy and i ^ 
have been in use for some time, and at first appears as a thin, depressed 
and flattened tooth, with rounded cutting edge and smaller in every 
dimension than h ; its principal diameter is transverse. It is the per- 
sistent growth of the tusk-like incisors (i- and i-3) with the diminution ol 
the others by abrasion, that so remarkably changes the appearance of the 
animal in passing from the youthful to the fully adult condition. In the 
stage in which i-3 is just erupted, all the incisors are closely crowded 
together and are arranged in imbricating fashion, each tooth being 
extensively overlapped by the one external to it. In older stages this 
overlapping is much reduced, partly by the growth of the jaw and widen- 
ing of the symphysis, partly by the narrowing of iy and 2, as they are 
worn down. 
The canine is a small tooth, of little or no functional importance, which 
is inserted somewhat internal to h and is usually in contact with the first 
premolar ; it has a narrow, compressed crown, with irregularly rounded, 
trenchant edge, convex on both internal and external sides, except that 
there is a shallow depression on the anterior portion of the inner surface. 
