EXTINCT UNGULATE MAMMAL EEOM PATAGONIA. 
177 
them, though developed to a much less extent than on the outer and inner faces of the 
crown. The grinding-surface of the tooth is a rather irregular four-sided area, broader 
externally than within ; it is composed of a smoothly worn surface of dentine with a 
thin enamel margin, and is deeply excavated longitudinally, the outer and inner margins 
standing out prominently, especially the former. Near the middle of this area, but rather 
towards the anterior and inner angle, is a very deep oval fossa, formed by an inflection 
of the enamel-covered outer surface of the tooth, extending in depth almost to the base 
of the crown, placed obliquely, the long axis of the oval directed from before backwards 
and inwards. The enamel of the outer margin of this fossa is plicated. 
The third and fourth premolars (p 3 and p 4) are formed on exactly the same prin- 
ciple as the one just described. They present respectively a slight increase in size, and 
the inner lobe of the crown becomes gradually rather broader, and its two pillars rather 
more widely separated from each other. The fourth, in addition to the principal oval 
fossa, has a second smaller one behind and to the outer side of it. All these teeth are 
wider from within outwards than from before backwards. 
In the first true molar (m 1) a considerable increase of the size of the crown takes 
place, especially in the antero-posterior extent. It is, however, formed upon precisely the 
same pattern as the hinder premolars, but merely expanded in the direction just indicated. 
Unfortunately only one of the upper true molars (the first of the left side) is pre- 
served in a complete state, and that, as might be expected, is considerably worn. There 
is also a broken first molar of the right side, and fragments of the second and third of 
the left. The most perfect tooth has a subquadrate crown, with a broad, flattened, or 
slightly convex outer wall ( l ), presenting several shallow vertical elevations and depres- 
sions. The most marked ridge (a e c) is very near the anterior edge of the tooth, and 
corresponds with the anterior external ridge of the premolars, and with the principal 
cusp of the canine and incisor teeth. The second and more posterior elevation (pec) 
is broader and far less salient. The inner wall has its two columns or ridges (aic and 
p i c) as in the premolars, and still somewhat converging as they approach the grinding- 
surface ; but they are wider apart and have a broad depression between them, which in 
the fragment of a nearly unworn more posterior tooth is seen to communicate with the 
central fossa. This fossa in the first molar is reduced to a very narrow but deep chink 
( m s), and there is no trace of the small second or posterior fossa of the last premolar. 
The outer wall of the crown is connected with the internal pillars by anterior and pos- 
terior transverse ridges (ar and pr), which pass one before and the other behind the 
median fossa or sinus. 
The lower teeth (figs. 2 & 3). — Of the lower incisors but one pair are present ; and as 
the symphysial portion of the jaw has completely perished, it is impossible to say what 
may have been the original number ; but analogy would lead to the inference that there 
were three, as in the upper series. The crown (i 3) is of an elongated oval form, with 
sharp cutting-edges, a strongly developed cingulum, and on the inner surface, msteade 
of a tubercle separated by a groove from the outer cusp, there is a vertical ridge 
MDCCCLXX1V. 2 A 
