266 : HORNS OF 777TANOTHERIUM CHAP. 
together with the long and divergent horn cores, must have given 
to the living animal a most bizarre appearance. It is an interest- 
ing fact that this animal, though a Perissodactyle, agrees with the 
Artiodactyla in the nineteen dorso-lumbar vertebrae, of which seven- 
teen bear ribs. . 
The genus further agrees with the Artiodactyles in the 
structure of the carpus. The toes of the fore-limb are four, 
those of the hind-limb three; but while the hind-lhmb is un- 
doubtedly Perissodactyle in the arrangement of its component 
parts, the fore-limb shows a hint of an Artiodactyle mode of 
structure. This hmb is paraxonic, the axis of the limb passing 
between the two middle digits. It may be that this genus 
represents more nearly than any other Perissodactyle or Artio- 
dactyle the primitive stem from which both have diverged, though, 
of course, it is not old enough to be very near to the actual 
ancestor. The molar dentition is the typical one; the incisors 
seem to vary as to their presence or absence, and, if present, in 
their numbers. In comparing the older with the more recent 
forms it is noteworthy that there has been an increase of size 
exactly as there has been during the evolution of the Camels and 
some other groups of Ungulates. As already mentioned, the size 
of the horn cores also increases until it culminates in the extra- 
ordinary species, 7. platyceras and T. ramosum, in which. these are 
half as long as the skull, flattened in form, and connected at 
their bases by a “web” of bone. Arrived at this amount of 
specialisation the genus 7itanotheriwm apparently exhausted its 
capacities for modification and ceased to be. The many 
generic names may be explained by sexual differences on the one 
hand and an incomplete knowledge of connecting links on the 
other. ' 
Palaeosyops is somewhat lke a Tapir in build, the skull 
especially resembling that of the Tapir. As in TVitanotheriwm 
the molar teeth, instead of having an outer wall formed by fused 
cusps, have a W-shaped outer wall on one side and two or one 
cusps on the opposite side. It is, moreover, an Eocene form, and 
in correspondence with its greater age is more primitive in some 
points of structure, for example, in the absence of horns and in 
the full dental formula. The fore-limbs are four-toed, the hind 
1 See especially Osborn and Wortman, Bull. Amer. Mus. Nat. Hist. vii. 1895, 
> 
p. 333, and Osborn, ibid. viii. 1896, p. 157. 
