1896.] MAMMALIAN DENTITION. 581 



with those teeth, but differed from them in being retarded in its 

 development. 



Wliile 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 d])m. 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 Sore.v 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 

 molai'iform — 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 Eats," 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 carnassial 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 tootb-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 me in a former paper (28), 

 still concludes that the successor to dpm. 4 is the true represen- 

 tative 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 



