The larger number of incisive and molar teeth among the Marsupials suggests that their additional teeth have disappeared in the Eutheria,[7] and Mr. O. Thomas has endeavoured to construct a generalised dental formula from which both the Marsupial and Eutherian modifications may have been derived by the suppression of particular teeth. Thus the hypothetical formula _i_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₅, _c_ ¹⁄₁, _p_ ¹,²,³,⁴⁄₁,₂,₃,₄, _m_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₅, by the loss of the fifth lower incisor, and of the second premolars (which we know to be those which disappear in the Marsupials) and the fifth molars, will give _i_ ¹,²,³,⁴,⁵⁄₁,₂,₃,₄,₀, _c_ ¹⁄₁, _p_ ¹,⁰,³,⁴⁄₁,₀,₃,₄, _m_ ¹,²,³,⁴⁄₁,₂,₃,₄; or the formula of the Opossum (_Didelphys_), usually written _i_ ⁵⁄₄, _c_ ¹⁄₁, _p_ ³⁄₃, _m_ ⁴⁄₄. Again, in the same formula the loss of the fourth and fifth incisors in both jaws, and also of the fourth molars, gives us _i_ ¹,²,³,⁰,⁰⁄₁,₂,₃,₀,₀, _c_ ¹⁄₁, _p_ ¹,²,³,⁴⁄₁,₂,₃,₄, _m_ ¹,²,³⁄₁,₂,₃, or the formula of a typical Eutherian, like the Pig, which we generally write as _i_ ³⁄₃, _c_ ¹⁄₁, _p_ ⁴⁄₄, _m_ ³⁄₃. Such a generalised formula will admit of modification into that of all existing, and a large number of fossil Marsupials, but it is possible that some of the Mesozoic types may have had more than four premolars, although there is no absolutely decisive evidence that such was the case. The presence of seven or eight true molars in some Mesozoic forms merely entails the addition of two or three additional figures to the ideal generalised formula. The milk-dentition of all known Marsupials, existing or extinct, is (if not entirely absent) limited to a single tooth on either side of each jaw, this being the predecessor of the last permanent premolar. And if the view that the milk-dentition is an additional series grafted upon the original permanent series be correct, it is evident that we have in this single replacement the first stage of this additional development. In very few mammals are teeth entirely absent. Even in the Whalebone Whales their germs are formed in the same manner and at the same period of life as in other mammals, and even become partially calcified, but they never rise above the gums, and completely disappear before the birth of the animal. In some species of the order Edentata, the true Anteaters and the Pangolins, no traces of teeth have been found at any age. The adult Monotremata are likewise devoid of teeth of the same structure as those of ordinary mammals; but well-developed molars occur in the young _Ornithorhynchus_, although no traces of teeth have hitherto been detected in _Echidna_. _Modifications of the Teeth in Relation to their Functions._—The principal functional modifications noticed in the dentition of mammalia may be roughly grouped as piscivorous, carnivorous, insectivorous, omnivorous, and herbivorous, each having, of course, numerous variations and transitional conditions.