AMPHITHERIUM (Thy/atOtheriUM, Val.)*—This genus is founded upon a few specimens of lower jaw, one ramus of which (fig. 85) gave the entire dentition of its side,—viz., three small conical incisors (i), one rather larger canine (c), six pre molars, unicuspid, with a small point at one or both sides of the base (p, 1-6), and six quinque-cuspid molars (m, z.6) not departing very far from the type above described. The mo lars, and most of the premolars, are implanted by two roots. The condyle of the jaw is convex, and is a little higher than the level of the teeth ; the coronoid process is broad and high ; the angle projects backward, with a feeble production inward. It is, again, to the marsupial Myrmeeoblits, amongst living forms, that the present genus is most nearly allied. The remains of Amphi therium are from the lower oolitic slates of Stonesfield (fig. 87, stratum s).
genus is founded on a ramus of the lower jaw, from the Stonesfield oolitic slate, showing true molars of a compressed form, with a large middle cusp and a smaller, but well marked, one at the fore and back part of its base ; the "cingulum," or basal ridge, peculiar to mammalian teeth, traverses the inner ridge of the crown, where it develops three small cusps, one at the base of the large outer or princi pal cusp, and the other two forming the anterior and posterior ends of the crown. This form of tooth is unknown in existing Mammalia, but is as well adapted for crushing the cases of coleopterous insects (elytra of which are found fossil in the same oolitic matrix) as are any of the multi-cuspid molars of small opossums, shrews, and bats. The Amphilestes Broderipii was somewhat larger than Arnphitherium Prerostii.
PHASCOLOTHERIUM.—Although the evidence of the very slight degree of inflection of the angular process of the lower jaw of Amphitheriam may favour its affinity to the placen tal Insectivores, yet the range of variety to which that mandi bular character is subject in the different genera of existing Marsupialia warns us against laying undue stress upon its feeble development in the extinct genus of the oolitic epoch, and incites us to look with redoubled interest at whatever other indications of a marsupial character may be present in the fossil remains of other genera and species of Mammalia that have been detected in the Stonesfield slate.
In the specimen of Phascolotherium (fig. 86) presented to the British Museum by William .T. Broderip, Esq., F.RS., its Lower jaw and teeth of the Phasoolotherium (nat. size in outline), Lower Oolite.
original describer,* which is as perfect in regard to the dentition as the jaw of the Amphitherium above described, the marsupial characters are more strongly manifested in the general form of the jaw, and in the extent and position of the inflected angle, while the agreement with the genus Didelphys in the number of the premolar and molar teeth is complete. The forms of
the crowns of those teeth differ from those in Didelphys, and correspond so closely with those in the A mphilestes Broderipii, as to show the closer affinity of the Phascolothere with the latter oolitic Insectivora ; and, accordingly, whatever additional evidence of marsupiality is afforded by the Phascolotherium, may be regarded as strengthening the claims of both Amphi lestes and Amphitherium to be admitted into the marsupial group. The general form and proportions of the coronoid process of the jaw of Phascolotherium resemble those in the zoophagous Marsupials ; and especially with that of the Thylacynus in regard to the depth and form of the entering notch between this process and the condyle.
The base of the inwardly-bent angle of the lower jaw pro gressively increases in Didelphys, Dasyurus, and Thylacinus ; and judging from the fractured surface of the corresponding part of the fossil, it most nearly resembles the jaw of Thyla cinus. The condyle of the jaw is nearer the plane of the inferior margin of the ramus in the Thylaeine than in the Dasyures or opossums : and consequently, when the inflected angle is broken off, the curve of the line continued from the condyle along the lower margin of the jaw is least in the Thylacine. In this particular, again, the Phascolothere resem bles that Australian Carnivore. In the position of the dental foramen, the Phascolothere, like the Amphithere, differs from the zoophagous Marsupials and placental Carnivora and Insect ivora, and resembles the Hypstprymnus, a marsupial Herbivore, that orifice being near the vertical line dropped from the last molar tooth. In the direction of the line of the symphysis, the Phascolothere resembles the Opossums more than the Dasyures or Thylacines. It is probable that the teeth at the fore part of the jaw showed the same correspondence. In the number of the molar series, the Phascolothere differs from Amphitherium, Amphilestes, and Myrmecobius, and resembles the Thylacine and Opossum, but without having the premolars (p, distinguished, as in them, from the true molars (m, 1, 2, 3, 4), by smaller and more simple crowns. As, however, these two kinds of teeth can only be determined by their order of development and succession, the Phascolothere may well have had three premolars and four true molars.