108 MICROSCOPIC ANATOMY OF THE TEETH 



independently of all these secondary curvatures, is from 

 the dentine to the enamel surface. In the Manatee the 

 prisms in many parts have a straight course from dentine 

 to surface, but this is the rarest condition in the enamel of 

 mammalia. The enamel of Rodents is in the majority of 

 species characterized by its arrangement on the inner side 

 in decussating layers, while on the outside the layers are 

 parallel to one another. This division into two layers was 

 pointed out by Professor Owen (12), but the most complete 

 research on the structure of Rodent enamel was carried 

 out by Sir John Tomes and described in a paper read before 

 the Royal Society in 1850 (17 b). This paper, the result of 

 a very complete investigation of the enamel of the different 

 genera and species of Rodents, showed that the structure 

 of the enamel was to be looked upon in many cases as an 

 assistance in the identification of species. In the words of 

 this author, he was enabled to show that ' the teeth of some 

 species of the order have specific characters by which they 

 can be distinguished from any of her known teeth '. ' That in 

 all Rodentia except the Leporidse (hares and rabbits) a portion 

 of the enamel has a laminated arrangement of its fibres '; 

 ' that the enamel laminae have a different and distinctive 

 character in each of the larger groups, and that the variety 

 of structure is constant throughout the members of the 

 same group.' The outer and inner portions of the enamel 

 in all Rodents (except the Leporidse) show two distinct 

 areas, the portion near the dentine exhibiting a decussation 

 or crossing of the laminae, and the outer portion a disposition 

 in parallel lines. In the different species there is a great 

 variation in the width of these layers, in the angle of inclina- 

 tion of the prisms to the dentine surface, and in the pattern 

 formed by the decussation of the inner layers. 



Castoridae. In the Beaver (Castor fiber) the decussation of the inner 

 layers is very distinct, as is also the change in the direction 

 of the prisms in the outer layer. In describing the 

 arrangement in the enamel of the Beaver, to avoid confusion 

 it may be explained that by a longitudinal section is meant 

 a section taken parallel to the long axis of the tooth, by 

 a transverse section one taken across the tooth at right 

 angles to the long axis, but of course in both instances the 



