no 



MR. W. BATESON ON 



[Feb. 2, 



series is relatively a small tooth, as, for example, ^ or m^ in the Dog, 

 then in cases of an addition to the series by which this terminal 

 tooth becomes the penultimate it will be found that this penultimate 

 tooth is larger and better developed than the corresponding ultimate 

 tooth of a normal animal of the same size. Of this phenomenon 

 two examples must here suffice. 



The first is a skull of Canis azarw in the Leyden Museum (fig. 3\ 

 p. 109). In this figure, I and II represent the back-teeth of this 

 abnormal specimen, I being the upper, and II the lower jaw. By 

 the side of these are shown the upper and lower jaws of a normal 

 specimen of rather larger absolute size. In the upper jaw there is a 

 supernumerary m^ on both sides and the great enlargement in m'^ and 

 especially in m^ is very striking. In the lower jaw there is no extra 

 tooth, but the molars are considerably larger than the normal size. 



The next case is that of Dasyurus maculatus, having an extra 



Fig. 4. 



Dasi/urus maculatus. 

 A. Eight upper jaw of a normal specimen. B and 0. Upper and lower jaws of 

 an abnormal specimen having an extra molar in each lower jaw and in 

 the left upper jaw. In correlation with this change the sixth upper 

 back-tooth (m'^) has been increased in size. (The abnormal specimen 

 was of less than normal size.) From specimens in the British Museum. 



molar in the left upper and both lower jaws. In this case, owing to 

 the great difference which normally exists both in shape and size 

 between the last tooth and the last but one, some obscurity is intro- 

 duced by the changes associated with the presence of an extra tooth, 

 and it would be difficult to determine the nature of the upper teeth 

 if this phenomenon of Variation were unrecognized. The structures 

 are shown in fig. 4, A being the right upper jaw of a normal 

 specimen, while B and C are the jaws of the abnormal form (Brit. 

 Mus. 983 b). The normal specimen is unfortunately much larger 

 ^ This drawing was kindly made for me by Mr. J. J. Lister. 



