1854.] OF THE ROYAL INSTITUTION. 371 
the sectorial or carnassial modifications, are not homologous or 
homotypal in the two jaws. 
In the genus Ursus the dentition was, in like manner, shown 
3—3 i Noret| 4—4 ha Bae wale 
BS ae ae) CoE) 
In the Hog, four deciduous molars are succeeded by four 
premolars, vertically; and three molars are developed in hori- 
zontal succession behind these, the dental formula being : — 
3—3 1—1 4—4 3—3 |" 
. » C. ———, p. ——, m. ——_ = 44, 
3—3 1—-1 4—4 3—3 
This number of teeth is never surpassed in the Diphyodont 
series ; and the Lecturer regarded it as the typical dentition. 
It is, however, rarely maintained in existing species, but appears 
to have been much more common in extinct Mammalia, es- 
pecially those from the most ancient tertiary epochs; illustrations 
of which were given in the Hyenodon, and Hyopotamus, and 
examples cited in the extinct genera Cheropotamus, Anthraco- 
therium, Hyracotherium, Oplotherium, Merycopotamus, Hippohyus, 
Anoplotherium, Paleotherium and Paloplotherium. In the three 
latter genera, Professor Owen had determined the nature of the 
molar series to be the same as in the Hog, by specimens shewing 
the deciduous dentition. 
In the hoofed quadrupeds with toes in uneven number (Peris- 
sodactyla), whose premolars, for the most part, repeat both the 
form and the complex structure of the true molars, such pre- 
molars are distinguished by the same character of development 
as those of the Artiodactyla, or Ungulates with toes in even 
number ; although here the premolars are distinguished also by 
modifications of size and shape. 
In most of the South American Quadrumana, the number of 
teeth as contrasted with the Monkeys of Africa and Asia, is in- 
creased to thirty-six, by an addition of one tvoth to the molar 
series on each side of both jaws. It might be concluded 4 priori, 
that as three is the typical number of true molars in the placental 
Mamunalia with two sets of teeth, the additional tooth in the New- 
World Monkeys would be a premolar, and form one step to the 
resumption of the normal number (four) of tnat kind of teeth. 
The proof of the accuracy of this inference was given by the state 
of the dentition in the young of the Howler-Monkey (Mycetes) in 
which a diagram was exhibited of a dissection of the jaws, expos- 
ing the germs of the permanent teeth: the crown of a premolar 
being found above the third milk-molar in place, as well as above 
the second and first. As regards number, therefore, the molar 
series, in Mycetes, is intermediate between that of the bear, Ursus, 
and Felis; the little premolar p. i. in Ursus, tells plainly enough 
which of the four is wanting to complete the typical number in the 
South American Monkey, and which is the additional premolar 
to be :— i. 
i 
