ATTRITION OF THE HYPSODONT TOOTH 



159 



Fig. 2. Longitudinal transparent section of sheep's tooth, about half worn, 

 showing plug of secondary dentine. Note thicker layer of enamel on labial (top) 

 surface. (X 5.) 



overlapping central incisors usually develop into a deep "V." Except 

 in grossly defective mouths, the permanent teeth are not necessarily 

 similar to the deciduous; and the mouths of twins are usually un- 

 related in type. Short lower jaw is the commonest mouth defect in 

 New Zealand sheep. This type of mouth wears down quickly, and 

 even the presence of this defect to only a mild extent, not usually 

 recognized as a fault by farmers, increases the rate of wear. 



These defects are probably all of hereditary origin and some are 

 congenital. The laws governing the inheritance of sliape and align- 

 ment of sheep's teeth are, as yet, little understood. The faults are 

 widespread, but by adopting a long-term policy of selecting desir- 



