1^0 
TERTJAET YERTEBRAT^V OF THE FAYtM. 
wliole of the posterior column, tliat is, according to the interpretation here adopted, 
the mesostyle, the postcro-external and postero-internal cusps. The small inner 
root is imperfectly divided by a vertical groove into a larger anterior portion 
supporting that ])art of the anterior main column homologous with the antero- 
intcrnal main cusp, and a smaller posterior portion supporting the portion of the 
cingulum {c.') which in wear unites the inner ends of the main columns. 
Tlie premolars (PI. V. figs. 2, 3) present a sharp contrast to the molars, their 
strikingly different appearance being due to the presence of an almost fiat outer wall 
which shows scarcely any trace of folding. Like the molars they attain a high degree 
of hypsodonty, particularly on their outer side, where the enamel-covered portion of 
the crown is almost four times as high as on the inner side (PI. V. fig. 3 a) : in 
consequence of this, as in the case of the molars, these teeth are strongly curved, 
the concave face being turned towards the palate (text-fig. 3, p. 11). The ectoloph 
is fiat or at most gently concave from before backwards, and it appears to be composed 
of three elements. Of these, one forms the antero-external angle of the tooth and 
is connected with the cingulum on the anterior face of tlie tooth, so that it may be 
regarded as a parastyle The element behind this is a large cusp (ae.) widened 
out transversely as it wears down, so that eventually it forms a junction Avith tlie cusp 
(«?’.) internal to it. Behind this main outer cusp is a smaller one forming the 
postero-external angle of the tooth. The anterior face of the tooth is occupied by a 
prominent crest of the cingulum (c.), which dies away internally, Avhile externally, as 
already mentioned, it is eonnected with the parastyle. On the inner face of the 
tooth there are two elements {ai. and ju.), the anterior of Avliich is the larger. This 
becomes connected Avith the antero-external cusp, forming a transverse crest, and at 
the same time it is united by a ridge Avith the postero-internal cusp. In still later 
stages of Avear this postero-internal cusp becomes united Avith the postero-external, 
so that finally the centre of the tooth is occupied by an enamel-lined pit enclosed by 
the continuous Avear-surfaces of the outer and inner pairs of cusps and tlie connecting 
transverse ridges (PI. V. fig. 3). 
The three posterior premolars have two roots, one anterior the other posterior, 
each extending the Avhole Avidth of the tooth : the division betAveen the roots is much 
deeper on the inner than on the outer side (PI. V. fig. 3 a). 
The anterior premolar is simpler than the others, having only one root. It is 
closely crowded between the second premolar and the canine, Avhich it much resembles, 
being a simple curved columnar tooth. The canine (PI. V. fig. 1) and the tAvo 
posterior incisors are similar and are also closely croAvded together. On their inner 
face there is a prominent cingulum enclosing with the croAvn a deep fosette. 
The anterior incisor, judging from its alveolus, is iiiucli larger than the others. 
In no specimen has this tooth been found in sifn, but an isolated examjile almost 
certainly referable to this position has a curved and pointed croAvii Avith a shelf-like 
