Heterodont

"On the Homologies and Notation of the Teeth of Mammalia," by W[illiam] H[enry] Flower, F.R.S. […] The classification and special homologies of the teeth of the heterodont mammals was next discussed.

The adaptation of the form of the teeth on both sides to this position, and the accurate adjustment of their contiguous surfaces, shows that it is a natural conformation. They are, moreover, of very nearly even height throughout the series, and in their configuration present a remarkable and gradual transition from the first incisor to the last molar, easily traced in both jaws, and more even and regular than in any other known heterodont mammal.

The few common characters by which the Edentata are associated are too well known to need repetition. The principal one is the abseance of any trace of the typical heterodont and diphyodont dentition, found in a more or less modified form in all other placental mammals.

[I]n many heterodont forms the teeth at the anterior end of the series of premolars and molars are small teeth, standing to the teeth behind them as the first terms of a series more or less regularly progressing in size.

The process of evolution of the specialised heterodont dentition of the Mammalia, or of the Theriodontia, from the simple homodont and polyphyodont dentition of the lower Reptilia would, I think, necessarily cause a reduction in number of the successional sets of teeth, due to an enlargement of one set and a consequent abstraction of growth, energy and material from the underlying sets.

Finally the structure of the palate in ?Acrodus (and to a lesser extent, Hybodus and Asteracanthus) shows certain general resemblances to that of modern heterodont sharks, particularly in the lower postorbital ramus, absence of an orbital process, well-developed ethmoidal articulations and absence of a basal angle.

Mammalian teeth are characteristically heterodont, i.e. teeth are dissimilar in shape and size.

The first cetaceans were heterodont and had multicusped cheek teeth. […] The earliest odontocetes were heterodont, and the earliest mysticetes retained teeth[…], though vascular grooves on their palates suggest baleen too[…].