No. 2.] THE HARD PARTS OF THE MAMMALIA. 259 
ple growth, resulting from use, in the direction of least resistance. 
Hence in teeth with opposite cusps, growth will connect them if 
anywhere, thus avoiding collisions with the corresponding growth 
in the opposite jaw. This is the primitive relation of cusps, and 
‘paonpar yonur {[eursi10 fopesojoD Jo paq JOA azY AA 
wo {soovjans Sueoyseu ‘uoyUap saoisayut pue solsadns ‘adod swcaz0uostg woposoguds — ‘1g IANSUT 
its result. In some of the Lophiodontidz (e.g. Systemodon) a 
slight alternation is observable in the cusps of the inferior 
molars, while those of the superior molar remain opposite. This 
alternation is developed to its fullest extent in Anchitherium, 
where the external and internal cusps of the superior molars are 
