150 
the penultimate lower milk molar, which in this as in most groups 
represents but the half of a true molar, furnishes opposition to its 
most anterior surface. Although it is not always literally true, that 
in the Artiodactyla the premolars represent each but the half of a 
true molar, and in the Perissodactyla an entire one, it is certain 
that in the exceptional cases among the former group, the parts re- 
presenting the posterior division of the tooth are small, or merely 
rudimental ; and that in the latter group, it is only in the most an- 
terior of the series that the posterior portion of the tooth is ever 
altogether wanting. It is also certain, that all those genera of which 
the milk dentition has been seen, conform in that particular to the 
general character, the distinction being well-marked in the Artio- 
dactyla between the two /as¢ upper milk teeth, whose characters are 
those of true molars, and those which precede them and represent but 
half ones, the same difference also prevailing between the /ast and 
those which precede 7¢ in the lower jaw ; always necessitating the ex- 
istence of a third pair of tubercles m the last lower milk molar to 
work in the interval of the two pairs in the penultimate above ; while 
in the Perissodactyla, the constant existence of a well-developed pos- 
terior pair of lobes in the penultimate lower milk tooth abrogates the 
necessity of a third pair in the last one, and consequently we need 
not expect to find it, even in those genera, such as Lophiodon and 
Paleotherium, of which the additional lobe to the last true molar is 
characteristic. Of the first-named genus, the milk dentition, so far 
as I am at present aware, is as yet unknown; but among the plates 
in the ‘Ossemens Fossiles’ examples may be seen of the lower jaws of 
young Paleeotheria, exhibiting the milk teeth, of which the last has but 
two lobes*. Therefore the tripartite condition of this tooth becomes 
a constant and important character of the Artiodactyle division. 
Most of the characters which separate the Ruminant and Non-ru- 
minant divisions of the Artiodactyla have been pointed out in my 
former paper, as well as those which distinguish the two subdivisions 
of the Hog-tribe, which by the analogy of the amount of difference in 
those of other groups, I think must be looked upon as families,— 
Suide and Hippopotamide. The striking character derived from the 
sudden termination of the pterygoid ridge in the Ruminant, and the 
formation of the pterygoid fossa in the other division, has been alluded 
to abovet+. The considerable upward extension of the masseteric ridge 
upon the os malz beneath the orbit seems also characteristic of the 
Ruminants, as well as the bifurcation of the orbital ala of the sphenoid, 
* Pl. 4. fig. 1 (alluded to by Professor Owen), and pl. 56. fig. 2. 
+ In the Hippopotamus the pterygoid ridge runs inwards and even a little back- 
wards, and then forms a slight angle at the point of junction with the pterygoid 
process, which then runs downwards and forwards, so that the outer wall of the 
fossa exists as in the allied forms, while, as I have before observed, it is the inner 
one which is wanting. I must again refer to the remarkable osseous bulla within 
the orbit of this animal, since I find that the same thing exists, though of much 
smaller size, in most ruminants; in many skulls it is broken away, and when re- 
maining it so lies upon the “tuberosity” or posterior termination of the alveolar 
process of the maxillary bone as to appear at first like a part of it. It opens into 
the nose and antrum maxillare, and has no connection with the lacrymal apparatus. 
