SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. I 1 9 
Pi, Ml. Supernumerary teeth occasionally appear among the premolars, 
and, in certain cases, some of the smaller teeth may be suppressed, but 
these appear to be abnormalities. 
A. Upper Jaw (Pis. XIII, XV, XVI, XVII, figs. 2, 3 ; XX, fig. 5).— 
The median incisor (i~) is a large tooth, which greatly changes in shape 
and appearance with age. When freshly erupted and unworn (PI. XVI, 
fig. 1) it is very broad and massive and the exposed portion is of uniform 
width; the masticating surface has an enamel-pit, or “mark” which, how- 
ever, is quite shallow, and its floor is irregularly pitted with numerous de- 
pressions, while the anterior and posterior borders are trenchant edges. 
On the anterior face the medial border is raised into a more or less prom- 
inent ridge and, external to this, is a broad, shallow concavity, while the 
outer moiety of the face is convex. Wear speedily obliterates the pit on 
the masticating surface and the concavity and convexity of the anterior 
face also soon disappear, for they have no great vertical extension, leaving 
that face gently convex, or nearly plane (PL XVII, fig. 2). For some 
time the base of this incisor remains open and the tooth continues to grow, 
the crown, in front view, having much the appearance of the scalpriform 
incisor of the rodents. Thick enamel covers the anterior face and is re- 
flected around the medial and external borders, but leaves the posterior 
face uncovered. With the formation of the root, which takes place when 
the animal is in early adult life, i 1 assumes quite a different appearance 
(PI. XVI, figs. 3-6), narrowing to the base and having a more or less tri- 
angular anterior surface. As growth has now ceased, the tooth becomes 
smaller with advancing age, as the crown is worn down by use. 
When the first incisor is erupted and for some time after it has come 
into use, there is still no visible trace of i-, di- being still in place, and 
for a considerable period after its eruption i- is much smaller and less 
conspicuous than i- (PI. XVI, fig. 3) but it grows from a persistent pulp 
and throughout the whole life of the animal, or, at least, to extreme old 
age, eventually far surpassing i 1 in size and especially in length. (Cf. 
PI. XVI, figs. 4-6.) It is these changes in the relative size of the median 
and second incisors which are the chief factor in the surprisingly altered 
appearance of the dentition in the successive stages of the animal’s devel- 
opment. I-, which thus becomes a formidable tusk, with considerable 
resemblance in shape to the lower canine of the boar, is recurved and 
sharply pointed and of trihedral cross-section, with the apex formed by 
