Amphilestes has teeth of the same pattern but has more of them, the premolars and molars being respectively four and five. All these animals had the lower jaw inflected. Whether they are all Marsupials or not, it is clear that Phascolotherium and Amphilestes should be united and placed away from Amphitherium on account of the more primitive form of their teeth.
We next come to the Trituberculata.
Among the most celebrated of these remains are a few jaws discovered in the Stonesfield slates near Oxford, and examined by Buckland, Cuvier, and some of the most eminent naturalists of the beginning of the last century. These jaws have been lately submitted to a careful re-examination by Mr. Goodrich,[1] who has increased our knowledge of the subject by exposing from the rocky matrix in which the jaws lie fresh details of their structure; it is probable therefore that now all that there is to be learnt from these specimens has been recorded.
Amphitherium prevostii was a creature about the size of a Rat. Its jaw was first brought to Dean Buckland about the year 1814, and described six years later. Buckland thought the jaw to be that of an Opossum, an opinion in which Cuvier concurred. The jaw, however, is marked by a groove running along its length, and this groove was regarded by de Blainville as evidence of the composition of the jaw out of more than one element, which would naturally lead to its being regarded as the jaw of a reptile.[2] This species and another named after Sir Richard Owen have a dental formula which, like that of the Marsupials, is large as compared with that of the Placental mammals; it runs: I 4, C 1, Pm 5, M 6—i.e. 64 teeth altogether. This is a larger number than we find in any existing Marsupial. But as in Marsupials, and in certain Insectivora also, the angle of the jaw is inflected. These teeth are of the tritubercular pattern with a "heel." They are in fact closely like those of the living Myrmecobius; but not, it should be remarked, unlike those of certain Insectivora.
The Mammals of the Cretaceous Period.—At one time there was a totally inexplicable gap between the Jurassic and the basal Eocene, a series of strata which occupy an enormous expanse of time in the history of the earth having appeared to
