1894 - 95 .] Dr J. Smith on the Mammalian Tooth. 341 
head. An apparent discrepancy in the development of the upper 
canine of the Babyroussa in this way presents itself. The tooth 
seems to spring from the upper instead of the lower or oral border 
of the superior • maxilla, and would thus suggest an inversion of its 
formative structure ; but in other members of the Suidse it has been 
shown that while the upper canine, at an early stage of its develop- 
ment, necessarily points downwards, the tooth, as it increases in 
length, curves upon itself, and, departing from the primary direction 
in the original axis of its alveolus, describes in its growth a circular 
sweep till its free extremity at last points upwards. And if it be 
kept in mind, along with this, that after the eruption of a tooth the 
alveolar walls continue to extend and adapt themselves alongside 
the lengthening fang, the difficulty is so far met, since the dental 
germ may have followed the usual course of development, while the 
subsequent mode of growth, accompanied by an altered form and 
direction of the alveolus, has produced in the Babyroussa what is 
possibly no more than an exaggerated example of similar peculiarities 
seen in other members of this family. There is another circum- 
stance connected with such curved teeth which also renders the 
axial twist less certainly determinable, and that is its being in their 
case frequently ill-marked and often abnormal, in so much that the 
tusk, instead of passing to the outer side of the head, occasionally 
inclines inwards and infringes upon or actually perforates the skulk 
The normal direction of the spiral, however, may be realised in a 
well-formed specimen of any of these exceptional organs by suppos- 
ing the tooth to be restored to the condition of a straight spire, and, 
in such as those of the Babyroussa, the alveolus and contained 
upper tusk to be completely inverted, when the sj)iral turn will be 
perceived to be the same as in the case of other animals, where the 
crown or exposed portion of the tooth appears as if it were rotated 
on its long axis somewhat backwards and inwards. 
In the Rodentia, the incisors are illustrative of the same pecu- 
liarities in form and construction. The combined free-cutting 
portion and the imbedded fang of these organs constitute a very 
much-prolonged tooth where the axial torsion is readily perceived, 
especially in the larger animals belonging to this order, such as the 
Capybara, the Porcupine, or the Paca, in which latter animal the 
twist is very apparent, the spiral winding in the same direction as 
