CHAP. X.] TEETH : SYMMETRY. 267 



hand the rarity of Variation in the dentition of the Common Fox 

 (C. vulpes} is noteworthy, especially when compared with the 

 extraordinary frequency of Variation in the molars of S. American 

 Foxes. The constant presence of the small anterior premolar in 

 the upper jaw of Otters (Lutra) of most species, as compared with 

 the great variability of the similar tooth in the Badgers (Meles} 

 and in other species of Otters, may also be mentioned. 



The evidence given in the last chapter should not, I think, be 

 taken as indicating the frequency of dental Variation in Mammals 

 generally. The orders chosen for examination were selected as 

 being those most likely to supply examples of the different forms 

 of dental Variation, and it is unlikely that the frequency met with 

 in them is maintained in many other orders. 



(2) Symmetry in Meristic Variation of Teeth. 



With respect to bilateral Symmetry an examination of the 

 evidence shews that dental Variation may be symmetrical on the 

 two sides, but that much more frequently it is not so. The in- 

 stances both of bilaterally symmetrical Variation, and of Variation 

 confined to one side are so many that examples can be easily 

 found in any part of the evidence. 



Besides these there are a few cases in which there is a variation 

 which is complete on one side, while on the other side the parts 

 are in a condition which may be regarded as a less complete 

 representation of the same variation. Such cases are Ommato- 

 phoca rossii No. 320, Phoca grcenlandica No. 324, Dasyurus macu- 

 latus No. 385, Canis lupus No. 24-6, G. vetalus No. 248, &c. 



In the remarks preliminary to the evidence of dental Variation, 

 reference was made to a peculiarity characteristic of the teeth 

 considered as a Meristic Series of parts. As there indicated, the 

 teeth are commonly repeated, so as to form a symmetry of images 

 existing not only between the two halves of one jaw, but also to a 

 greater or less extent between the upper and lower jaws. It was 

 then mentioned that cases occur in which there is a similar Varia- 

 tion occurring simultaneously in the upper and lower jaws of the 

 same individual. Such similar Variation may consist either in 

 the presence of supernumerary teeth, or in the division of teeth, 

 or in the absence of teeth. It should, however, be noticed that 

 examples of Variation thus complete and perfect in both jaws are 

 comparatively rare. Speaking generally, it certainly appears from 

 the evidence that similar Variation, (1) on one side of both jaws, 

 or (2) on both sides of one jaw and on one side of the other, or 

 (3) on both sides of both jaws are all rare. Of these three the 

 following examples may be given : 



Of (1), Macacus rhesus No. 190, Ateles pentadactyl us No. 196, 

 Esquimaux dog No. 243, Phoca vitidina No. 329. 



