It would be beyond the scope of the present work to describe in detail, or even to mention the names of all the members of this group, and it will therefore suffice to refer to a few of the principal types. Of the forms with tubercular premolars the best known is the genus _Tritylodon_ (Fig. 24), which occurs typically in beds of Lower Mesozoic in South Africa, but is also known from the Trias of Stuttgart. In the Stonesfield Slate, near Oxford, which belongs to the lower part of the Jurassic system, and is separated from the Trias by the intervening Lias, a fragmentary jaw with three teeth (Fig. 27) appears to indicate an allied type, the teeth having three longitudinal ridges separated by grooves. In the Purbeck beds of Dorsetshire, forming the top of the Jurassic system, we find another member of this group, which has been described as _Bolodon_, closely allied to which is _Allodon_ of the Upper Jurassic of the United States. [Illustration: FIG. 26.—The imperfect right ramus of the mandible of _Plagiaulax minor_; from Swanage. Four times natural size. _p_, Premolars; _m_, molars. (After Lyall.)] The first discovery of the remains of Mesozoic mammals was made in the Keuper or Upper Trias of the Rhætian Alps in Bavaria. In 1847 Professor Pleininger of Stuttgart, while sifting some sand from the Keuper of Diegerloch and Steinenbronn, found, among an immense mass of teeth, scales, and unrecognisable fragments of skeletons of fish and saurians, two minute teeth, each with well-defined, enamelled, tuberculated crowns and distinct roots, plainly showing their mammalian character. These were considered by their discoverer to indicate a predaceous and carnivorous animal of very small size, to which he gave the name of _Microlestes antiquus_. Subsequently Mr. C. Moore discovered in a bone bed of Rhætic (topmost Trias) age, filling a fissure in the Mountain Limestone at Holwell, near Frome in Somersetshire, various isolated teeth with their crowns much worn, but apparently including both upper and lower molars and a canine, which are assigned by Sir R. Owen to Pleininger’s genus _Microlestes_, and described specifically as _M. moorei_. Under the name of _Hypsiprymnopsis rhæticus_, Professor Boyd Dawkins described a single tooth with two roots discovered in the Rhætic Marlstone at Watchet in Somersetshire. Sir R. Owen referred the latter tooth to _Microlestes_, and if its describer is right in regarding it as a much worn premolar of the type of those of _Plagiaulax_ (Fig. 25) there would be evidence that _Microlestes_ was closely allied to the latter, from the molars of which those of _Microlestes_ are scarcely distinguishable. [Illustration: FIG. 27.—_Stereognathus oölithicus_. Fragment of jaw with three teeth (_a_, _b_, _c_), in matrix; from the Stonesfield Slate. Natural size. (After Owen.)]