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Show 1896.] MAMMALIAN DENTITION. 591 compose the order Insectivora, we find a considerable variation their dentition, both as regards the number of their teeth and the specialization of the individual members of the dental series. Thus in Gymnura and Talpa we find in the adult the full placental dentition of 44 teeth, while in two Shrews (Biplomesodon and Anurosorex) the dentition is reduced to 26 teeth, other families presenting numerous stages intermediate between these two* If primitive, the supposed presence of 4 upper incisiors in Sorex and the 4 upper molars of Centetes must be of great interest, but the former I believe is capable of being interpreted differently, and the latter to be a secondary character. A closer examination of these dental variations shows that they can be grouped under four heads :- (1) A tendency for a suppression of the 3rd incisor above and below, di. 3 disappearing first. (2) A suppression in the premolar series, pm. 1 in the Cen-teticlce, pm. 2 in Selenodon. (3) A suppression of the posterior molars, the number varying from |- to f, the normal number being -|. (4) A tendency for reduction in the functional importance of the milk dentition. Although representatives of only 5 out of the 9 families of the Insectivora (Plower and Lydekker, 4) have been systematically examined, this last variation is so marked, that one is forced to the conclusion that the order as a whole is tending to lose its milk-teeth. Among the forms examined, probably only Ericulus and Echinops possess the same number of functional milk and permanent antemolar teeth, but these forms have already a reduced dentition. Of those provided with 44 teeth, viz. Gymnura and Talpa, we find in the former ' ' p ' reduced and functionless, while in Talpa, omitting the 1st premolar, all the remaining milk-teeth are reduced and though cutting the gum can hardly function (if at all) for more than a week or two. The remaining genera examined show this reduction in a varying degree, the maximum being attained in Sorex, where in all probability the entire milk series is reduced and functionless. If then it be a fact, as is now generally believed, that the milk dentition preponderates in the early Mammalia and in the living Marsupials, then we must come to the conclusion that the living Insectivora are specialized forms tending towards a Monophyodont condition in which the preponderating dentition is the replacing or permanent set. LIST OF REFERENCES. 1. BATE, S.-"On the Dentition in the Mole." Trans. Odont. Soc. 1865-67, p. 261. la. BRANDT, E.-Ueber d. Zahnformel der Spitzmause. St. Petersburg, 1878. |