The most interesting fact about the molar series in the Theriodontia is that we meet with the two types of molars that occur in the mammals. _Cynognathus_ and other genera have molars which consist of a main cusp, and of one cusp before and one after the main cusp; in fact these teeth are triconodont as in certain early mammals, a state of affairs which is believed by the "trituberculists" (see p. 56) to have preceded the tritubercular tooth. There are also "multitubercular" teeth, especially well developed in _Tritylodon_, where they exactly resemble those of certain Multituberculata, and whose structure originally led to the placing of _Tritylodon_ among the mammals of that group. If there is any question about the mammalian nature of this fossil, there remain several other Theriodontia in which the multituberculism is well marked. It is so in _Trirhackodon_ {93} and in _Diademodon_ for instance. This incidentally lends some support to the idea that the Mammalia have been evolved from two sources, a way of looking at the origin of the group that will coincide with the views of some authors like the late Dr. Mivart, and will at the same time reconcile the trituberculists and the multituberculists. For we should then assume that the Eutheria and Triconodontia had originated from some such form as _Cynognathus_; and the Multituberculata and the existing Monotremes from some form like _Diademodon_. It is not of great use to point out that _Diademodon_ is really of the trituberculate pattern, because in its molars, though multituberculate, the trituberculate main cones can be recognised; for that state of affairs could just as well have been brought about by a reduction from the multituberculate type. The skull of these Theriodonts shows some well-marked approximations to the mammalian type. There is in the first place a commencing consolidation and reduction of the individual bones, which is so distinguishing a feature of the mammalian skull as opposed to the skull of lower vertebrates. In _Cynognathus_ the postorbital is fused with the jugal, and the supratemporal with the squamosal, forming apparently one bone. In the lower jaw the splenial is often reduced to the thinness of paper, thus indicating a commencing disappearance. In many Theromorpha the squamosal shares largely in the formation of the articular facet for the lower jaw, obviously an important mammalian characteristic; this is brought about by the reduction of the quadrate, which latter bone, moreover, acquires in certain particulars the appearance of the mammalian malleus, with which it is, according to many, homologous. But this subject has been already dealt with on page 26. A very pronounced likeness to the mammalian skull is that there are two occipital condyles. That this has been brought about by the further development of a tripartite condyle such as occurs in tortoises, and that by the suppression of the basi-occipital part, does not affect the