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Show 1896.] MAMMALIAN DENTITION. 581 with those teeth, but differed from them in being retarded in its development. While investigating the development of dpm. 4 and ppm. 4 in the Insectivora, I have kept the above conclusion in mind, and allowing for the differences in the condition of the dentition in these two groups (Insectivora and Diprotodont Marsupials) I find a strong confirmation of this view, that ppm. 4 represents a tooth originally situated in front of dpm. 4, but retarded in its development, and subsequently displaced backwards or overgrown by dpm. 4. This condition is more marked in the upper jaw, where in three of the genera investigated ppm. 4 develops distinctly in front of dpm. 4, in two slightly so, while only in one does it develop distinctly lingual to dpm. 4 (this is in Sorex probably a specialized form). The molariform condition of dpm. 4 is well marked, but while in some Insectivora ppm. 4 is distinct in pattern, in others it is also molariform-the former condition being more marked in other groups of mammals, in some of which (Carnivora and Marsupials) ppm. 4 is so distinct in the characters of its crown from its predecessor that, taken in connection with the developmental features above recorded, I am forced to the conclusion that dpm. 4 is a true molar accelerated in its development and growing forwards over the top of the retarded true 4th premolar, or, in other words, dpm. 4 is the only true deciduous molar, while the tooth usually termed ppm. 4 is really the milk, but non-deciduous 4th premolar. The above would account for the striking differences in character between the supposed deciduous and permanent 4th premolars of the "Kangaroo Rats," where dpm. 4 is molariform, and ppm. 4 that marvellous compressed cutting-tooth, identical in pattern with the anterior premolar dpm. 3. So also in the Carnivora with regard to the upper earnassial tooth. I think it is easier to conceive that the anterior molar should be accelerated in its development in order to supply the young animal with a crushing-tooth, than to believe with Cope (2) that the mere fact of a tooth-germ being shifted in its position relative to the angle of the mouth would cause such a total change in the character of two tooth-germs which were supposed to develop side by side as sisters from the same region of the dental lamina. It is only fair to state that Leche (9. pp. 103 and 139) after considering the views put forward by m e in a former paper (28), still concludes that the successor to dpm. 4 is the true representative of that tooth in the permanent series. The Molars. I have already described in my detailed account of the development of the molar teeth the presence of outgrowths from the dental lamina, to which structure the enamel-organs on these teeth are attached and from which they have arisen, both of the labial and lingual side of these teeth ; these outgrowths, though |