All the features in which the last two deciduous teeth, both above and below, are described as differing from their functional counterparts in the permanent dentition, are features found in the permanent teeth of primitive fossil mustelids and certain fossil and Recent viverrids. Even so, taking into account Leche's (1915) work, which shows that the milk teeth of some carnivores have structures lacking in the corresponding permanent teeth of the same individual animal and also in the teeth of genera that seem to be ancestral, a person suspects that some of the structural features mentioned above are not inheritances of ancestral conditions but rather specializations of the milk dentition. [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