it was first thought that the dentary contained three parallel rows of teeth. instead of arranging the teeth in longitudinal rows, they are now known to fall into obliquely arranged zahnreihen. in each zahnreihe, the anteriormost tooth is the oldest and the posterior most tooth is the youngest. there is active ongoing replacement of these tooth rows. the distal portion of each tooth is compressed from side to side and is somewhat pear shaped in cross section.