Allied types, such as _Ctenacodon_, are also met with in the Upper Jurassic of North America; and the _Plagiaulacidæ_ also persisted into the lower part of the Eocene division of the Tertiary period; _Neoplagiaulax_ being a Tertiary form common to Europe and the United States, while _Liotomus_ and _Ptilodus_ are at present known only from the latter country. The present group is also represented in the upper Cretaceous of the United States by _Selenacodon_ (_Meniscoëssus_ in part), _Cimoliomys_, etc. _Polymastodon_, of the Lowest or Puerco Eocene of New Mexico is the largest known form, and is characterised by the presence of only one premolar and the elongated molars. The angle of the mandible is inflected after the Marsupial fashion. _Polyprotodont Types._—The second type of mammalian dentition found in the Mesozoic period resembles that occurring among recent Polyprotodont Marsupials—that is to say there are at least three lower incisors, the canines are well developed, and the premolars and molars are cuspidate, the number of the latter reaching in some cases to seven or eight. There has been much discussion as to the taxonomic position of these forms, and while the majority of writers admit the Marsupial affinities of at least a moiety, it has been contended that others indicate distinct ordinal groups more or less closely allied to the Insectivora. At present, however, there is no decisive evidence to support such a view. Important proof of the Marsupial affinity of one of these forms is afforded by the replacement of the teeth, which appears to be of the same nature as in the existing Marsupials, that is to say, the last premolar alone is preceded by a milk-tooth. The most generalised forms appear to be _Dromatherium_ and _Microconodon_, from Lower Mesozoic beds in the United States, of which enlarged views of the teeth are given in Fig. 4 (1, 2), p. 31. Professor Osborn points out the extremely simple character of these teeth, and it is quite possible that these forms may prove to be _Prototheria_. There are three premolars and seven molars in the lower jaw of _Dromatherium_. [Illustration: FIG. 28.—Reversed view of the left ramus of the mandible of _Triconodon mordax_; from the Purbeck of Swanage. Natural size. (After Owen.)]