The difference between these teeth in the lower jaw of Didclphys is shown by the addition, in the true molars, of a pointed tubercle on the inner side of the middle cone. In Phascolotherium a mere basal ridge or cingulum extends along the inner side of the middle cone. Such a ridge is present in the last molar of Sarcophilus, but not in the other molars ; but in these there are two small hind cusps on the same transverse line, whilst that cusp appears to be single in Phascolotherium. The cingulum, moreover, in the second to the penultimate of the molar series of this fossil, extends so far as to form a small talon at the fore and back part of the crown ; thus making five points, which are very distinct in the third to the penultimate tooth inclusive ; and by this character the dentition of Phasco lotherium differs materially from any existing Marsupial, and repeats the type of molar which, as yet, would seem to be peculiar to the Insectivora of the oolitic epoch. There is a feeble indication of this structure in the antepenultimate and penultimate molars of Thylacinus, but the hinder division of the crown shows two small cusps on the same transverse line, besides the rudimental hindmost one ; and there is no cin gulum. Upon the whole, it would seem that, though the affinity may not be close, Phascolotherium most resembles Thylacinus amongst existing Mammals ; but Thylacinus is now confined to Tasmania, and is there fast verging to extinction.
The resemblance shown by the lower jaw and its teeth of the Aniphithere and Phascolothere to marsupial genera now confined to Australia and Tasmania, leads one to reflect on the interesting correspondence between other organic remains of the Oxfordshire oolite and other existing forms now confined to the Australian continent and surrounding sea. Here, for example, swims the Cestracion, or Port-Jackson shark, which has given the key to the nature of the "palates" from our oolites, now recognized as the teeth of congeneric larger forms of eartilafrincninfinbcts_ Mr. Broderip, in his Memoir above quoted, observes, "that it may not be uninteresting to note that a recent species of Trigonia has very lately been discovered on the coast of Australia, that land of marsupial animals. Our speci men lies imbedded with a number of fossil shells of that genus." Not only Trigonia but tula exist, and the latter abundantly in the Australian seas, yielding food to the Cestracion, as their (After Fitton.) extinct analogues 1. Rubbly limestone (cornbrash).
a. Clay, with Terebratnlites.
doubtless did to the 3. Limestone rock.
allied Plagiostomes 4. Blue clay.
5. Oolitic rock.
with crushing teeth, 6. Stiff clay.
called Acrodus,Peam- 7. Oolitic rag, or limestone.
modus, etc. Araw 8. Sandy bed containing the Stonesfield elate.
saris and cycadeous plants, like those found fossil in oolitic beds, flourish on the Australian continent, where marsupial quadrupeds now abound ; and thus appear to complete a picture of an ancient condition of the earth's surface, which has been superseded in our hemisphere by other strata and a higher type of mammalian organization. Fig. 87 repre sents a section of the strata overlying the slates whence the fossil mammalian jaws, with associated Megalosaurs, Ptero dactyles, and other oolitic organisms, have been obtained at Stonesfield in Oxfordshire. The vertical thickness of the strata
through which the shaft is sunk to the gallery is 62 feet; on the side opposite the right hand is marked the depth of the horizon tal gallery, where the slate is dug which contains the fossils ; on the opposite side the strata are numbered in succession.
Genus STEREOGNATHUS.—The last evidence of a mamma lian animal discovered in the Stonesfield slate is of peculiar interest, because it exhibits a type of grinding teeth quite dis tinct from any of the previously acquired jaws from that lo cality, and affords evidence of a small vegetable-feeding or omni vorous quadruped. It consists of a portion of a lower jaw, im bedded in the characteristic matrix (fig. 88), about 9 lines in extent, and containing three molar teeth (a, b, c). It is nearly straight ; the side exposed is convex vertically ; a slight bend downwards, and decrease of vertical diameter towards the end, indicates it to be part of a left ramus. This is unusually shal low, broad or thick below, the side passing by a strong convex curve into the lower part ; a very narrow longitudinal ridge, continued after its subsidence by a few fine lines, forms a tract which divides the lateral from the under surface ; else where the bone is smooth, without conspicuous vascular perforations. The depth or vertical diameter of the ramus is not more than two lines. Of the three teeth remaining in this portion of jaw, the middle one is the least mutilated. The crown of this tooth (fig. 89, B) is of a quadrate form, 3 milli metres by 3i millimetres, of very little height, and supports six subequal cusps in three pairs, each pair being more closely connected in the an tero-posterior direction of the tooth than transversely.
The outer side of the crown (fig. 88, b), supported by a narrow fang which con tracts as it sinks into the socket, shows two principal cusps or cones, and a small accessory basal cusp. The hard and shining enamel which covers these parts of the crown contrasts with the lighter cement that coats the root. The two outer lobes or cones are subcompressed, and placed obliquely on the crown, so that the hinder one (o, fig. 89) is a little overlapped externally by the front one o, the fore part of the base of the hinder one being prolonged inwards on the inner side of the base of the front cone. The two middle cones i) are subcompressed laterally, with the fore part of their base a little broader than the back part. The two inner cones (p, pi) have their inner surface convex, with their summits slightly inclined forwards. The fore part of the base of the hinder cone is prolonged obliquely towards the centre of the crown, beyond the contiguous end of the base of the front cone, so as to cause an arrangement like that of the two outer cones (o, o'), the obliquity of the posterior cone of both the outer and the inner pairs being such that they slightly converge as they extend forwards.