THE DIGESTIVE AND RESPIRATORY SYSTEM 279 



straight line following 

 the line of the jaw, 

 is called triconodont, 

 one in which the bend- 

 ing has taken place is 

 trigonodont. This tri- 

 tubercular tooth, still 

 under the influences of 

 the opposing tooth sur- 

 faces, may develop a 

 lateral spur, the hypo- 

 cone, upon which 1-3 

 tertiary cups, the co- 

 nidi, may develop, or 

 else the hypocone may 

 form the point of a pro- 

 jecting spur, the talon. 

 The nomenclature given 

 here is that of the up- 

 per teeth, the corre- 

 sponding parts of the 

 lower jaw being distin- 

 guished by the addition 

 of the suffix -id, thus : 

 protoconid, hypoconid, 

 talonid, etc. In favor 

 of this theory may be 

 urged its complete ap- 

 plicability to all known 

 forms, both living and 

 fossil, as a system of 

 nomenclature, and its 

 correspondence in se- 

 quence of stages to that 

 shown by the extinct 

 forms in consecutive 

 geological periods. The 



rt 



FIG. 76. Phyletic history of the molar 

 cusps. [After H. E. OSBORN.] 



I. Reptilian stage, haplodont; Permian. II. 

 Triconodont stage. III. Tritubercular stage. 

 IV. Tritubercular-tuberculo-sectorial ; lower Ju- 

 rassic. V. The same, upper Jurassic. VI. The 

 same, upper Cretaceous. VII. The same, Puerco, 

 lower Eocene. VIII. Sexitubercular-sexitubercu- 

 lar, Puerco. IX. Human lower molar. 



Pt, protocone; P, paracone; m, metacone; h 

 hypocone; Ptd, protoconid; Pd, paraconid; mi, 

 metaconid; hd, hypoconid; hid, hypoconulid; ed, 

 entoconid. 



