494 professor' OWEN ON THE DENTITION OF PHACOCHOERUS, 
odonts three of the seven on each side of both jaws are ‘ premolars/ four are ‘ true 
molars.’ In the placental Diphyodonts four of the seven on each side of both jaws 
are ‘premolars,’ and three are ‘ true molars.’ The exceptions, by way of excess to this 
typical number, are few and are confined to the marsupial order, e. g. in the Myrme- 
cobius : the deviations from the type by deficiency are numerous, especially in the 
placental series. But to whatever point the number may be reduced, the teeth that 
are retained may be identified with their homologues in the typical series. This 
power is fortunately given by the constancy in respect of their existence of the con- 
tiguous teeth of the premolar and molar series, those, e. g. marked p 4 and w 1 in 
Plate XXXIII. ; and by the absent teeth being taken from a definite part of each 
of those series, viz. from the fore part of the premolars and from the back part of 
the true molar series. 
The Bears and some Carnivora offer a partial exception in the occasional retention 
of JO 1 when p 2 and p 3 may be absent, but jo 4 is constantly present. 
Thus it needs only to determine, in any given species of diphyodont mammal, the 
last premolar and the first true molar, as has been done in the young Phacochere, 
Plate XXXIII., in order to know the homologies of the rest. The true molars are 
counted from before backwards, — ‘ first,’ ‘second,’ ‘ third :’ the premolars from behind 
forwards, — ‘fourth,’ ‘ third,’ ‘ second,’ ‘first,’ or as far as the series may extend. In Man, 
€. g. it stops at the third ; the first and second, which exist in the Hog, being absent, 
but all the true molars are present. The teeth being thus determined their symbols 
can be applied to them, m I, m2, m3, for the molars, — p A, p3,p2, p\, for the pre- 
molars. It needs only to apply the symbols to one side of each jaw, the teeth being 
symmetrically repeated on the opposite side : and in most cases they are alike in both 
jaws. The right canine is the homotype of the left canine, as the right arm is of the 
left arm, agreeably with the principle of ‘ bilateral symmetry:’ the first true molar in 
the lower jaw is the homotype of the first true molar of the upper jaw, as the leg is 
the homotype of the arm, in accordance with the law of the composition of the verte- 
brate framework of a successive series of essentially similar segments. In whatever 
direction or to whatever degree two or more of these segments may deviate from the 
type, the same elements may be discerned in them beneath those modifications. If 
the neural arch be vastly expanded, as in the cranial vertebrae of mammals, we trace 
the broad and bifid neural spine from one to the other, and recognise, e. g. the frontal 
bones and parietal bones as homotypal elements. If the diverging appendages be the 
seat of adaptive development, as in the occipital and pelvic vertebrae, we find the 
same plan of modification is followed, and we can trace the homotypal parts, e. g. 
humerus and femur, radius and tibia, ulna and fibula, carpus and tarsus, as also the 
homotypal ossicles in the carpus and tarsus, even when the common plan is so varied 
in such appendages, as to produce the different powers and functions which charac- 
terize the leg and the arm in Man. 
So, likewise, Avhen two costal arches are converted into jaws and made to support 
teeth, we find the same laws of development so strictly followed as to enable us to 
