ENAMEL 111 



arrangement without some such device, but it may be much better 

 understood by a rough model. If some wooden matches are strung on 

 a wire or pin exactly through the middle of their length, and are then 

 alternately placed to form a series of rectangular crosses, two arms of 

 which are placed on a level surface, the appearance of the inner layer in 

 a transverse section is represented. To allow for the obliquity of the 

 prism the crosses must be inclined backwards from the base line at an 

 angle of 60. Another match attached to the upper ends of the arms 

 of the cross and sloped backwards at an angle of 70 to the base line 

 represents a prism of the outer layer. If this model is viewed sideways 

 the appearance in longitudinal section is at once explained. The cross 

 laminae leaning backwards at an angle of 60 are seen in the longitudinal 

 view as parallel lines at an angle of 60 to the base line, and the piece 

 representing the prism of the outer layer slopes at an angle of 30. The 

 cut ends of the crossing prisms which are directed obliquely forwards 

 are not very evident in the Beaver, but are better seen in some other 

 Rodents, as the Squirrel. 



A transverse section of the enamel of the Squirrel (Sciurus) Sciuridae. 

 shows a similar rectangular crossing of the laminae near the 

 dentine, but this inner layer extends across two -thirds of 

 the width of the enamel and the crossing laminae are at 

 right angles, or upright to the dentine surface, and when 

 viewed in longitudinal section the laminae are seen to be 

 arranged at right angles to the dentine surface, not inclined 

 at an angle of 60 as in the Beaver. 



In the Squirrel, as previously stated, the cut ends of the 

 crossing laminae can be distinctly seen in many longitudinal 

 sections. 



The prisms of the outer layer in Sciurus are bent at an 

 angle of 45 from the direction of the crossing laminae, 

 and are arranged in a single layer almost at right angles 

 to the enamel surface, but are very slightly curved, the con- 

 vexity of the curve being directed towards the cutting edge 

 of the tooth. In the Squirrel there is a very thin layer of 

 enamel, its width being according to a measurement by Sir 

 John Tomes only ^-gtli of an inch, and he also speaks of 

 the colour as confined to the outer third ; but in a specimen 

 from which the photograph, fig. 62, was taken the deep 

 orange colour extends more than half-way across, gradually 

 fading, however, towards the dentine. 



There are very slight differences in the arrangement of the 

 prisms in other members of the family, but, as the author 



