[Illustration: FIGS. 2-9. Views of permanent and deciduous teeth of _Mustela frenata nigriauris_. Incisors not shown. In each instance teeth are of the left side.
Permanent dentition × 3. No. 32421, Mus. Vert. Zoöl., [M], adult; Berkeley, Alameda County, California; obtained October 4, 1921, by D. D. McLean.
Deciduous dentition × 5. No. 132158, U. S. Nat. Mus., [M], juvenile; Stanford University, Santa Clara County, California; obtained May 7, 1898, by W. K. Fisher.
Figs. 2-3. Lateral views of upper teeth, of adult and juvenile respectively.
Figs. 4-5. Occlusolingual views of upper teeth of adult and juvenile respectively.
Figs. 6-7. Lateral views of lower teeth of adult and juvenile respectively.
Figs. 8-9. Occlusolingual views of lower teeth of adult and juvenile respectively.]
In other deciduous teeth there is clearer evidence of more specialization for a diet of flesh in the deciduous teeth than in the permanent teeth. For example, the upper carnassial of the milk dentition is even more highly sectorial than is the permanent tooth and strikingly like that of some of the cats. The lower tooth that is effective in the shearing action bears no more trace of the metaconid than does the permanent first lower molar. These features of the deciduous dentition suggest that it is more specialized for a diet of flesh than is the permanent dentition. If this be the fact, it may seem especially remarkable because the commonly employed term "milk teeth" suggests that the animal makes but little or no use of these teeth in the short time that they are in place. Accordingly, the student may credit the form of these teeth more to some indirect effects of inheritance than to natural selection acting directly upon the teeth. But, after all, natural selection probably is responsible for the form of these teeth as is indicated by the observations of Hamilton (1933:318-325). He found that these milk teeth are used for eating solid food as soon as the principal shearing teeth are in place. This is three weeks after birth and before all of the deciduous teeth have broken through the gums. These shearing teeth are used for almost two months before being replaced by the permanent teeth and it is, therefore, evident that natural selection could operate to fully as great a degree in determining the form of the deciduous teeth as it may with the permanent teeth.