The second mammiferous genus discovered in the same slates was named originally by Mr. Broderip Didelphys Bucklandi (see [fig. 293.]), and has since been called Phascolotherium by Owen. It manifests a much stronger likeness to the marsupials in the general form of the jaw, and in the extent and position of its inflected angle, while the agreement with the living genus Didelphys in the number of the premolar and molar teeth, is complete.[270-A]
On reviewing, therefore, the whole of the osteological evidence, it will be seen that we have every reason to presume that the Amphitherium and Phascolotherium of Stonesfield represent both the placental and marsupial classes of mammalia; and if so, they warn us in a most emphatic manner, not to found rash generalizations respecting the non-existence of certain classes of animals at particular periods of the past, on mere negative evidence. The singular accident of our having as yet found nothing but the lower jaws of seven individuals, and no other bones of their skeletons, is alone sufficient to demonstrate the fragmentary manner in which the memorials of an ancient terrestrial fauna are handed down to us. We can scarcely avoid suspecting that the two genera above described, may have borne a like insignificant proportion to the entire assemblage of warm-blooded quadrupeds which flourished in the islands of the oolitic sea.
Mr. Owen has remarked that as the marsupial genera, to which the Phascolotherium is most nearly allied, are now confined to New South Wales and Van Diemen's Land, so also is it in the Australian seas, that we find the Cestracion, a cartilaginous fish which has a bony palate, allied to those called Acrodus and Psammodus (see [figs. 307], [308.] [p. 275.]), so common in the oolite and lias. In the same Australian seas, also, near the shore, we find the living Trigonia, a genus of mollusca so frequently met with in the Stonesfield slate. So, also, the Araucarian pines are now abundant, together with ferns, in Australia and its islands, as they were in Europe in the oolitic period. Many botanists incline to the opinion, that the Thuja, Pine, Cycas, Zamia, in short, all the gymnogens, belong to a less highly developed type of flowering plants than do the exogens; but even if this be admitted, no naturalist can ascribe a low standard of organization to the oolitic flora, since we meet with endogens of the most perfect structure in oolitic rocks, both above and below the Stonesfield slate, as, for example, the Podocarya of Buckland, a fruit allied to the Pandanus, found in the Inferior Oolite (see [fig. 294.]), and the Carpolithes conica of the Coral rag. The doctrine, therefore, of a regular series of progressive development at successive eras in the animal and vegetable kingdoms, from beings of a more simple to those of a more complex organization, receives a check, if not a refutation, from the facts revealed to us by the study of the Lower Oolites.
Fig. 294.
Portion of a fossil fruit of Podocarya magnified. (Buckland's Bridgew. Treat. Pl. 63.) Inferior Oolite, Charmouth, Dorset.
The Stonesfield slate, in its range from Oxfordshire to the north-east, is represented by flaggy and fissile sandstones, as at Collyweston in Northamptonshire, where, according to the researches of Messrs. Ibbetson and Morris, it contains many shells, such as Trigonia angulata, also found at Stonesfield. But the Northamptonshire strata of this age assume a more marine character, or appear at least to have been formed farther from land. They inclose, however, some fossil ferns, such as Pecopteris polypodioides, of species common to the oolites of the Yorkshire coast[271-A], where rocks of this age put on all the aspect of a true coal-field; thin seams of coal having actually been worked in them for more than a century.
Fig. 295.