268 



MAMMALIA. 



A 



' y-v ^^ _ ^^ _ 



O O O 



o-O o 



j> VAVAV 



FIG. 157. 



DlAGBAM TO ILLUSTRATE THE TBITUBEECULAR THEORY OF CUSP-DEVELOPMENT 

 IN THE TEETH OF THE MAMMALIA. (After Osborn.) 



The cusps of the upper teeth are represented by round black dots, those of the 

 opposing lower teeth by small circles. 



A. Eeptilian stage, the diagram indicating four simple conical upper teeth, 

 with their similar lower teeth alternating but somewhat inside. 



B, C. Triconodont stages, three teeth and two teeth respectively. 



D. First triangular or tritubercular stage, three upper and two lower teeth. 



E, F, G. Stages with upper molars still simply triangular, the lower molars 



gradually developing a heel. 



H, I. Stages in which both upper and lower molars develop a heel. 



J. Stage exhibiting upper molars with four cusps, lower molar with five cusps. 



