154 
Having now summed up as much of my series of observations with 
regard to the Artiodactyle division as I think it at present expedient 
to offer, I proceed to consider the Perissodactyle group. 1 observe 
that Prof. Owen separates the Proboscidia as a third group, to which 
he seems to assign a rank equivalent to that of the other two, and 
passes the Deinotherium aud the Towodon, as well as the ‘‘ Sirenoid”’ 
forms, with some remarks which do not assign to them any very defi- 
nite location. There will always be room for difference of opinion as 
to the rank that should be assigned to a group, even when its limits 
are fully recognised ; since, as I have elsewhere endeavoured to show*, 
* granting affinities and even groups to be natural, the limits assigned 
to those degrees of difference and similarity which we are wont to in- 
dicate by definite terms are not ;” but it seems to me, that although 
these more aberrant groups of Ungulata possess several peculiarities 
which are entirely their own, they do not differ from the Perisso- 
dactyla in essential characters to the same degree as the latter do 
from the Artiodactyla, while in certain respects they agree among 
themselves, as though they would constitute a second subdivision of 
the Perissodactyla again divisible into strongly marked families. 
Among the characters which I have brought forward, we find that 
the Proboscidia, the Sirenia, and the singular fossil genus Towodon, 
agree with the more typical Perissodactyla in the depth of the inter- 
maxillary bone and the vertical implantation of the incisors, in the 
absence of the supraorbital foramen, of the fossa and pit within the 
orbit, and of a strongly marked pterygoid ridge, in the character of 
the zygoma, except that in the Proboscidia there is no descending 
post-articular process ; in the narrowing anteriorly, and rounded sides 
of the basioccipital bone, and in the resemblance between the anterior 
and posterior molares. They differ from the typical Perissodactyla 
and agree among themselves, in the upward direction of the nasal 
opening, the large size of the infraorbital foramen, the lengthening 
of the bony palate, with the comparative narrowing of the posterior 
nares, in the short antero-posterior extent and the transverse thicken- 
ing of the pterygoid processes, and in the considerable angle formed 
between the basioccipital and basisphenoid bones (least marked in 
the Manatee), the latter being inclined upwards, of course with refer- 
ence to the upward direction of the nasal canal. Points of resem- 
to the upper molars of the Dichodon seems to have crossed the mind of Prof. 
De Blainville, for in a recent number of the ‘ Ostéographie,’ after describing the 
dentition of the lower jaw in that animal, he proceeds: “ D’aprés ce qui vient 
d'étre dit du systeme dentaire de cette mandibule, on voit qu’il est incomplet par 
Vabsence de Ja derniére molaire non encore sortie; mais ne doit-il pas en étre de 
méme pour la série d’en haut, si les deux piéces proviennent du méme individu? 
Alors il faudrait admettre qu’au lieu de deux, il ne manquerait qu’une seule avant- 
molaire, ce qui parait peu probable.” 
With regard to the Hyopotamus Vectianus, M. De Blainville seems to doubt a 
littie that the upper and lower jaw really belong to each other, but refrains from 
a decided judgement, not yet being acquainted, as he observes, with any principle 
that can direct the mind in the question of the relation of two parts of the dental 
system to each other. He inadvertently calls this species “ annectens,”’ the name 
given by Prof. Owen to his Paloplotherium. 
* Essay on Classification, ‘ Zoologist’ for December 1847. 
