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Show 558 MB. M. P. WOODWARD ON [JVIay 5( one for those teeth, then we find this type apparently preserved all its purity in certain living Insectivores (Centetes, Ericuhs, and others), a condition almost unique amongst living mammals; in addition, some Insectivores exhibit molar teeth which are supposed to be but slightly in advance of this, having acquired a small heel above and below, thus presenting to us the trituberculo-sectorial type (well seen in the upper molars of Tupaia, Sorex, &c, and in the lower molars of the Centetidce and Chrysochloris). On the other hand, in many respects the dentition of this order cannot be regarded as primitive, for the ante-molar teeth are obviously specialized both with regard to their form and number. The molars, too, in many genera are clearly modified from a tritubercular standpoint, the upper molars being often quinque-tubercular, while below the heel may attain equal importance with the trigon and develop numerous cusps ; in others the paraconidis lost, thus producing a quadritubercular crown, an admittedly specialized type of lower molar. As a whole the teeth of this order are characterized by the strong development of their cusps, a condition closely associated with their insectivorous diet; this, perhaps, accounts for their resemblance to the teeth of the early Jurassic mammals, it being highly probable that the latter were also insectivorous. If this was the case, then the presence of these supposed primitive tooth-patterns among living Insectivores may be due rather to the similar nature of the food of these two groups, so widely separated iu time, than to an actual persistence of the unmodified tritubercular molar from Mesozoic times until to-day. Until recently it was generally supposed that the Insectivora were quite normal in their tooth change, Owen (18), Eousseau (21), Dobson (3), and others describing a full milk-dentition in some genera. But at the same time, it was known, from the researches of Spence Bate (1) on the Mole (Talpa), that the milk-dentition might be very transitory. Recently Leche (7 & 9) has published the results of an investigation concerning the relationships of the milk and permanent sets of teeth in a number of genera, adopting the more modern methods of microtomy to aid him in his researches, which were extended to foetal as well as numerous stages after birth, until the full adult dentition was acquired. In his first and preliminary communication, Leche (7) came to the most interesting conclusion that in the anterior tooth-region of the adult Erinaceus a mixture of milk and successional teeth was to be met with. The adoption of these results unfortunately led m e to put forward the view (30) that Erinaceus, in respect to the relation of its sets of teeth, was intermediate between the marsupial condition with its persistent milk set and the typical diphyodont placental stage. This now turns out to be quite erroneous, for Leche, in his later and complete work (9), shows conclusively that Erinaceus possesses vestiges of two complete dentitions, and that those anterior teeth, which are apparently only |