1847. ] OWEN ON ENGLISH EOCENE MAMMALIA. 33 
is equal to the antero-posterior extent of the last two premolars. In 
Paleotherium crassum (Cuv. 1. c. pl. 53. fig. 1) the same space 
measures 44 lines (1 centimeter), or little more than half the antero- 
posterior extent of the last premolar: and the proportions of the. 
diastema are nearly the same in the other known species of typical 
Palzeotheres. | 
The antero-posterior extent of the oval outlet of the socket of the 
canine (fig. 1, c) is 45 lines (9 millimeters). This socket is sepa- 
rated from the outer incisor (¢ 3) by a diastema of 21 lines (5 mil- 
limeters). The three subequal incisors are juxtaposed, and the inner 
one (2) is close to the premaxillary symphysis. 
The canines seem to have been rather smaller proportionally than 
in the typical Palezotheres, but there is no specimen either of these 
or of the incisive teeth by which the characters of their crowns can 
be given; their relative size and position are indicated only by the 
sockets. 
If we compare now the characters of the dentition of the upper 
jaw with those of the under jaw of the Hordle Palzeotherioid previ- 
ously described, we shall find that they agree with each other both in 
the antero-posterior extent of the molar series and in that of each indi- 
vidual tooth. They further exhibit the more important concordance 
in the number and kinds of those teeth, viz. p wae S m = the 
two first teeth (P 2 & 3, fig. 1) show the same comparative simplicity 
of structure, and consequently the same deviation from the character 
of those teeth in the Paleotherium; and if the other grinders of the 
lower jaw differ less from their homologues in Paleotherium and less 
resemble those in Anoplotherium than do those of the upper jaw, we 
shall not find that this opposes any real difficulty to their association 
with such upper jaw, when we observe that the differential characters 
between Paleotherium and Anoplotherium are much less marked in 
the lower than in the upper grinders ; just as the difference between 
Cheropotamus and Dicotyles is less marked in the dentition of the 
lower than in that of the upper jaw. The extent of deviation there- 
fore, though small, in the true molars of the lower jaw of the Hordle 
Paleeotherioid from those of the true Paleotherium, accords with or is 
proportional to those greater differences which we observe m the true 
molars of the upper jaw*. When however we find superadded to the 
degree of concordance between the true molars of the upper and lower 
jaws of the Hordle Paleotherioid, and to their differences from those 
teeth in the Paleeotheres and Anoplotheres, a common manifestation 
of the more striking and important differences in the minor number and 
simpler structure of the premolars, no reasonable doubt can be enter- 
* The bony arch supporting the upper molars being more remote from the centre 
of life than that supporting the lower molars, being in fact the anterior terminal arch 
of the hzemal series, is the seat of a greater amount of adaptive variation than the 
succeeding arch, and the teeth which it supports are in like manner ‘more varied. 
This principle is well-illustrated by comparing the upper molars of the odd-toed 
Ungulates generally with the lower molars of the same natural group. 
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