710 



DR. E. LoXXBEHG OX THE 



[June 19, 



have on their anterior surface a layer of enamel which, measured 

 on a longitudinal section about | mm. from the tip, has a thickness 

 of 400-435 fi. On the posterior surface the enamel, measured at 

 the same distance from the tip, has also a considerable thickness, 

 namely about two-thirds of that of the anterior layer (tin. 9). This 

 offers a striking contrast to the condition prevailing in the common 

 Calf and the Lamb. In an unborn lamb the anterior layer of enamel, 

 measured near the tip of the incisor, is 330 p. in thickness, but 

 the posterior layer is at the middle of the crown only 66 /i or one- 

 fifth of the anterior one, and is less than half that thickness 



Kg. 9. 



Section through the tip of a milk-incisor of a Musk-ox. 



or not even one-tenth of the anterior layer. In a new-born 

 common calf the incisors are provided with a layer of enamel on 

 their anterior side which in thickness near the tip equals that of 

 the Musk-calf (about 400 /< or more; (cf. fig. 10, p. 71 1). But on the 

 interior surface the layer of enamel is very thin and becomes towards 

 the tip only 20-10 /j. Such- a thin layer is, of course, very soon 

 worn off. The interior layer of enamel of the milk-incisors in 

 Sheep, and still more in Cattle, can thus with full right be termed 

 rudimentary. This condition is of course secondary ; and Ovilos, 

 with its fully developed layer of enamel, represents a more 



