SPALACOTHERIUM, Ow.—The next stratum overlying the older oolites in which mammalian remains have been detected, is a member of the newest oolitic series at Purbeck, Dorsetshire, called the " marly " or " dirt-bed." In a series of fossils discovered there by Mr. W. R Brodie, and transmitted for determination in 1854 to the writer, amongst the remains of fishes and small Saurians, constituting the majority of the specimens, were detected three unequivocal evidences of a mammalian species, which were described under the name of Spalaeotherium* trieuspiden& The specimen here selected (fig. 90) to exemplify the above extinct insectivorous Mammal, is a right ramus of the lower jaw. The posterior half contains four teeth, and extends backward beyond the dental series ; but instead of showing the compound structure which that part of the jaw exhibits in the lizard tribe, it con tinues undivided ; the convex surface showing a smooth depression for the insertion of the temporal muscle ; the lower boundary answering to that going to the condyle and angle of the jaw, and the upper one to that going to the coronoid process in the ramus of the jaw of the mole and shrew. The crowns of the teeth are long, narrow, and tricuspid, the inner part of the crown being produced into a point both before and behind the longer cusp which forms the chief outer division of the crown. Each of these teeth is implanted by a fang divided externally into two roots, in a distinct socket in the substance of the jaw. The multicuspid crown, the divided root of the tooth, its complex implantation, and the undivided or simple structure of the ratans of the jaw, all concurred, therefore, to prove the mammalian nature of this fossil.
The other specimens showed that the Spalacotheriurn had ten molar teeth in each ramus of the lower jaw, preceded by a small canine and incisors. The anterior molars are compressed, increase in height and thickness to the sixth, and from the seventh decrease in size to the hindmost, which seems to be the last of the series. The sharp multicuspid character of so
much of the dental series as is here preserved, repeats the general condition of the molar teeth of the small insectivorous Mammalia in a striking degree : one sees the same perfect adaptation for piercing and crushing the tough chitinous cases and elytra of insects. The particular modification of the pointed cusps, as to number, proportion, and relative position, resembles in some degree that of the Cape mole (Chrysochlora aurea), but both in these respects and in the number of molars, the dentition accords more closely with that of the extinct Amphitherium. The chief interest in the discovery of the Spedacotherium is derived from its demonstration of the exis tence of Mammalia about midway between the older oolitic and the oldest tertiary periods.
Both the Oxford oolitic slate and the Purbeck manly shell beds give evidence of insect life ; in the latter formation abun dantly. The association of these delicate Invertebrata with remains of plants allied to Zamia and Cycas, is indicative of the same close interdependency between the insect class and the vegetable kingdom, of which our power of surveying the phenomena of life on the present surface of the earth enables us to recognize so many beautiful examples. Amongst the numerous enemies of the insect class ordained to maintain its due numerical relations, and organized to pursue and secure its countless and diversified members in the air, in the waters, on the earth and beneath its surface, bats, lizards, shrews, and moles now carry on their petty warfare simultaneously, and in warmer latitudes work together, or in the same localities, in their allotted task. No surprise need therefore be felt at the discovery that Mammals and Lizards co-operated simultan eously and in the same locality at the same task of restraining the undue increase of insect life during the period of the de position of the Lower Purbeck beds.