No. 2. ] THE HARD PARTS OF THE MAMMALIA. 255 
them. When they join the internal cusps and form crests, the 
latter are either obliquely transverse to the anteroposterior di- 
ameter of the crown (most of the genera), or directly trans- 
verse (Tapiridze), or longitudinal (Equidz). The causes effecting 
these changes may be now considered. 
In the Tapiridz the absence of any noteworthy lateral move- 
ment of the lower jaw has prevented the development of cres- 
cents on the crowns of the molars of either jaw. The fact that 
the intermediate tubercles of the upper jaw are opposite the ex- 
ternal ones has led to the development of transverse crests in 
both jaws. The development of these crests must be regarded 
as the result of simple growth, which has pursued the direction 
of least resistance. The growth has been stimulated by the 
fore-and-aft mutual contact of the tubercules of each set of 
teeth on the other. 
In the case of the genera with alternate intermediate cusps 
in the molars of each jaw, the connecting crests when present 
have naturally become oblique. We notice that from the primi- 
tive type of Hyracotheriinz, two lines possessing this character 
have had their origin: those of the rhinoceroses and those of 
the horses, the latter commencing with the Lambdotheriidz. In 
the former the external crest of the superior molars do not form 
V’s; in the latter they do (Figs. 80, 81). In both, intermediate 
tubercles have been developed in lines anterior to the apices of 
both the external and internal cusps. It results from this that 
when these join the inner tubercles, they form parts of crests 
which reach the external wall of the crown anterior to the cor- 
responding external cusps. 
As the intermediate tubercles alternate with both internal 
and external cusps in the upper molar, it follows that the ex- 
ternal cusps of the inferior molars in passing from within, 
across the superior molars, pass obliquely over the summits of 
the former. This draws out the anterior external angle of the 
tubercle so as to form its prolongation as a crest, to the notch 
between the external cusps, as in Anchitherium (Fig. 83). 
When this crest is completed, the posterior oblique branch of 
the inferior V in vertical motion shears on its anterior edge as in 
other Ungulates, differing only from the tapirs in being oblique 
instead of transverse. In transverse horizontal motion, the lower 
crest crosses it at an oblique angle. In the Equine series the 
