TOOTH-GERMS IX A ^YALLABY. 939 



of a supernumerary incisor in the upper jaw ot" an adult Perameles 

 in the teaching collection of the Royal College of Science in which 

 thei'e are " indications of at least three fangs, and is obviously a 

 fusion of (these) teeth." The possibility of tooth fusion in the 

 mammalia is therefore by no means improbable. The matter 

 must remain unsettled until more satisfactoi-y evidence is forth- 

 coming, but the condition figured would seem to point in that 

 direction and we can only repeat that we can suggest no other 

 explanation. 



IV, General Considerations. 



■ A. Sericd homologies of the Teeth. 



Upper Incisors. — Assuming our identification of the teeth to be 

 correct, we have six incisors present in the upper jaw, of which 

 the 5th is unrepresented in the earlier stage and has attained but 

 a slight degree of development in Stage II. This excessive 

 number of upper incisors confirms the original statement of M. F. 

 Woodward (11), as the result of his observations on Petrogale 

 peuiciUata and Afacropus glgantens. That author is obviously 

 aware of the difficulty in accepting so large a number, for he writes 

 (p. 465): "The discovery of six pairs of incisors, although an abso- 

 lute fact, is in many respects an unfortunate one, as we know 

 of no adult mammal with so many, and even amongst Reptiles, 

 many Lizards and Crocodiles have the number of teeth in each 

 premaxilla restricted to five."' He then proceeds to discuss various 

 possible explanations such as the late calcification of the functional 

 incisors in Macropus and the greater space in the pi'emaxillary 

 region in Petrogale as the possible determining causes. A reference 

 to our text- figures 187 &^ 189 (pp. 933 & 936) will show that 

 Woodward has possibly found the correct explanation, both 

 factors appearing to be present in our specimens. 



The difficulty is, however, not fully stated by a mere reference 

 to recent forms, for Osborn (2) in his paper, " On the Structure 

 and Classification of Mesozoic Mannnalia," considers that the 

 dental formula of the primitive heterodont mammal should l">e 

 taken as i. 4, c. 1, pm, 4, m. 8. 



In a paper published three years later (13) Woodward appears 

 to have modifieil his former opinion, for in spite of having pre- 

 viously spoken of the presence of six upper incisors as " an absolute 

 fact," he writes " I am now disposed to regard the three minute 

 teeth which I described in the upper jaw of several genera 

 belonging to this family " (i. e. Macropodidje) " as the premilk 

 predecessors of the three functional incisors." He gives no reasons 

 for the alteration in his opinion, as in his previous paper he dis- 

 cusses and discards the interpretation which he appears later to 

 have adopted. He further makes no reference to the alteration 

 in the identification of the homologies of the functional incisors 

 which such a change of opinion would involve. 



We have naturally therefore paid particular attention to these 



