PERISSODACTYLA 369 
in size and structure. Crown of the last lower molar commonly 
bilobed, and if a third lobe is present in this tooth it is wanting in 
the last lower milk-molar. Dorso-lumbar vertebrae never fewer than 
twenty-two, usually twenty-three in the existing species. Nasal 
bones expanded posteriorly. An alisphenoid canal. Femur with 
a third trochanter! The middle or third digit on both fore and 
hind feet larger than any of the others, and symmetrical in itself, 
the free border of the ungual phalanx being evenly rounded (see 
Fig. 151). This may be the only functional toe, or the second and 
fourth may be subequally developed on each side of it. In the 
Tapirs and many extinct forms, the fifth toe also remains on the 
fore limb, but its presence does not interfere with the symmetrical 
arrangement of the remainder of the foot around the median line 
of the third or middle digit. Traces of a hallux have only been 
found in some extremely ancient and primitive forms. The 
astragalus has a pulley-like surface above for articulation with the 
tibia, but its distal surface is flattened and unites to a much greater 
extent with the navicular than with the cuboid, which bone is 
of comparatively less importance than in the Artiodactyla. The 
calcaneum does not articulate with the lower or distal extremity of 
the fibula. The stomach is always simple, the cecum is large and 
capacious, the placenta diffused, and the mamme are inguinal. 
The gall-bladder is invariably absent. 
As regards the dentition, the whole of the premolar series 
may be preceded by milk-teeth ; and it has been demonstrated in 
Rhinoceros that when there is no displacement of the first cheek- 
tooth that tooth is a persistent milk-molar; the same condition 
apparently holding good in Paleotherium. This feature indicates 
considerable dental specialisation, the milk-molars, according to the 
theory generally accepted by the leading English zoologists, being 
the acquired, and the premolars the original series. Another 
peculiar feature of the dentition of the Perissodactyla, very rarely 
met with among the Artiodactyla, is that the premolars tend to 
resemble the true molars ; this feature occurring in all the existing 
genera, although not found in the earlier generalised types. The 
cheek-teeth of all the members of the suborder are primarily con- 
structed on some modification of what is known as the lophodont 
plan. Thus the upper molars (Fig. 155, p. 375) have an outer antero- 
posterior wall from which proceed two transverse ridges, formed by 
the coalescence of the primitive inner and outer columns, towards 
the inner aspect of the crown; while in the lower molars there 
may be either two simple transverse ridges, or these ridges may be 
curved into crescents, coming into contact with one another at their 
extremities. Those forms having brachydont teeth show this plan 
of structure in its simplest modification; but in cases, as in the 
1 Wanting in the aberrant Chalicotherium. 
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