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1924] Loomis, Miocene Oreodonts in the American Museum
3
suppression of certain crests, or crescents, is characteristic of phyla and
genera. In some cases a further modification is present in the development
of a cusp-like pillar at the junction of the median crest and the
posterior crescent. The fourth upper premolar appears like a halfmolar,
and this peculiarity is attained differently in different phyla,
making this tooth and its origin of prime importance in studying artiodactyls.
A few cases will illustrate this.
Merycoidodon
cut bertsoni'
P6ebrothe.rium
wilsoni
Blistomeryx
primus
Aqrioch6erus
lact ifrons
Fig. 2. Upper premolars, left side, of an oreodont, a camel, a deer and an agriochoerid,
to show differences in character and mode of development.
In the oreodonts, examining the premolars from the first to the
fourth, it will be seen that premolar 4 corresponds to the posterior half
of premolar 3, the posterior crescent being greatly developed and united
to the median crest, and all trace of the anterior part of the tooth has
disappeared, except for a tiny pit in the anterior outer corner, which is
the vestige of the outer part of the anterior basin seen on premolar 3.
In a few very primitive genera, traces of both parts of the anterior basin
are retained; and in the more advanced genera, all trace of the anterior
half of the tooth is lost. See figure 4.
In contrast with this, the fourth premolar of camels is formed by
shortening the whole of premolar 3; its inner crescent is composed of