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Show 1892.] NUMERICAL VARIATION IN TEETH. 113 of the series have been met with. Cases of this kind are known 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. Ateles 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 Museum a unique pair of jaws in both of which a very similar tessellation 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 b) 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 P R O C ZOOL. Soc-1892, No. VIII. 8 |
