1892.] 



NUMERICAL VARIATION IN TEETH. 



113 



of the series have been met with. Cases of this kind are known in the 

 tessellated teeth of Rhinoptera (an Eagle-Ray), the most remarkable 

 being that described and figured in R.jussieui by Smith Woodward 

 in Ann. & Mag. N. H. ser. 6, vol. i. 1888, p. 281. In this specimen 

 (fig. 5, upper diagram) the number and arrangement of the teeth were 

 quite different on the two sides, those on the one side being normal, 

 while those on the other were unlike any known form. This new kind 

 of tessellation was, nevertheless, so regular and definite that had it 

 been existing on both sides the specimen would undoubtedly have 



Fig. 6. 



-^f^liaiO^y " 



Wl" K,^ 



Aides marginatus, having four premolars on each side in the upper jaw instead 

 of three. The lower jaw is normal, and the lower canine consequently 

 bites behind the upper. (Brit. Mus. 1214 b.) 



been made the type of a new species. There is, indeed, in the British 

 Museum a unique pair of jaws in both of which a very similar tes- 

 sellation occurs in a nearly symmetrical manner, and though this 

 specimen has been described as R. polyodon, it is by no means unlikely 

 that it is actually a " sport " of this kind derived from the usual 

 formula of Rhinoptera. 



One case in Mammals may be mentioned. This is a specimen 

 of Ateles marginatus (Brit. Mus. 1214 h) having four premolars 

 in each upper jaw instead of three (fig. 6). The lower 

 jaw is normal. There is nothing in this specimen to indicate 



Proc. Zool. Soc— 1892, No. VIII. 8 



