XIII] OF HORSES' TEETH 907 



The "horn" or tusk of the narwhal is a very remarkable and 

 a very anomalous thing. It is the only tooth in the creature's head 

 to come to maturity; it grows to an immense and apparently 



Kg. 446. Enamel patterns (diagrammafic) of certain fossil Equidae. 

 A, Protohipptcs ; B, Hyohippus; C, Neohipptis. 



unwieldy size, say to eight or even nine feet long; it never curves 

 nor bends, but grows as straight as straight can be — a very singular 

 and exceptional thing; it looks as though it were twisted, but really 



Fig. 447. Enamel pattern (diagrammatic) of the upper molar teeth of Rhinoceros. 

 The back-tooth (to the right-hand side) is the least worn, and its contour-line lies 

 at the highest level. 



carries on its straight axis a screw of several contiguous low-pitched 

 threads; and (last and most anomalous thing of all) when, as 

 happens now and then, two tusks are developed instead of one, one 

 on either side, these two do not form a conjugate or symmetrical 

 pair, they are not mirror-images of one another, but are identical 

 screws, with both threhds running the same way*. 



* The male narwhal carries the horn, the female being tuskless; but the whalers 

 say that the rare two-horned specimens are ull females. A famous two-horned 

 skull in the Hamburg Museum is known to have belonged to a pregnant female. 

 It was brought home in 1684, and is one of the oldest museum specimens in the 

 world; the tusks measure 242 and 236 cm. During my thirty years' close 

 acquaintance with the Dundee whalers, only four two-horned narwhals passed 

 through my hands. Bateson {Problems of Genetics, 1913, p. 44) makes the curious 

 remark that "the Narwhal's tusks, in being both twisted in the same direction, 

 are highly anomalous, and are comparable with pairs of twins." 



