456 ME. M, F. WOODWARD ON MAMMALIAN DENTITION. [May 2, 



animals. As may be seen (fig. 29) the 3rd incisor (t^) develops in a 

 backwai'd prolongation of the preiuaxilla situated on the palatal 

 side of the maxilla and side by side with the canine, and only attains 

 its true position in yery advanced pouch-specimens. 



The overcrowded state of the jaw will further explain the 

 complete atrophy of the already reduced 2ud, 3rd, and 5th incisors. 

 And to the same cause we may, I think, ascribe the abnormal 

 position of these teeth having become displaced by the larger ones 

 till they have assumed a position with regard to the latter which 

 suggests the I'elation between teeth belonging to the 1st dentition 

 and their successors in tlie 2nd dentition. 



The true reason for the small size of these teeth is probably to 

 be found in their disuse together with an early enlargement of the 

 1st and 4th incisors. This last character would according to 

 Cope's mechanical theory (2) be enough to account for their entire 

 disappearance; he suggests that the enlargement of any organ 

 causes the abstraction of the growth-energy and material from 

 some adjacent structure, as in the case of tho enlarged canines 

 of the AVart-hog causing a reduction of tho upper incisors. In 

 trying to account for these reductions in the number of teeth every 

 feature which can possibly bear on the case should be taken into 

 consideration, as it is more probable that a number of factors 

 have acted together to bring about these results, rather than 

 they can be accounted for by one only. It is interesting to 

 note in the light of Cope's suggestion that in this Petrogale, where 

 we find a greater number of these vestigial teeth than in any other 

 Macropid which I have examined (except M. giganteus), the adult 

 incisors are of a much slighter nature than in the latter forms, 

 where some of these vestigial teeth have either completely dis- 

 appeared or if present have become much reduced. I think that 

 the presence of these additional teeth is to be explained rather on 

 the above grounds, than on the supposition that Petrogale penicillata 

 is an extremely primitive form. 



The presence of the remains of six pairs of upper incisors is 

 altogether unexpected and rather unfortunate, for one would hardlj' 

 expect to find in so specialized a family as the Macropodidse more 

 incisors than are present in the least specialized Polyprotodonts, 

 In these latter forms with their 5 upper incisors, however, the 

 premaxilla in the foetus is very much crowded, more so than in 

 Macroims. As the teeth in the former all develop at the same 

 time, whereas in the latter, as we have seen, they develop progres- 

 sively from before (backwardly), this may perhaps account for the 

 suppression of the 6th incisor. Judging from the condition of the 

 adult in Peramehs, I should imagine that the first incisor to 

 disappear was number 5, as there is a large diastema in the adult 

 between the 4th and 5th incisors. This suggestion was further 

 strengthened by the study of a series of sections through the jaw 

 of a young Peranieles, while the premaxilla was still short and 

 consequently crowded with teeth : at this stage a slight but definite 

 thickening of the dental lamina (fig. 2S) was observable between 



