ANCESTRY OF THE MAMMALIA 57 



in the last which projects more than the first molar. It should 

 also be noted that in Trieonodon the fourth postcanine tooth 

 alone is replaced by a successor. 



It was from Triconodont mandibles that Cope many years 

 ago formulated his theory of a migration of the lesser cones 

 from their original position to form ultimately a triangle with 

 the larger cusps. This view is not held at the present day but 

 in view of its historic importance as an early enunciation of 

 the Theory of Trituberculy we shall recapitulate the two 

 propositions in the original presentation of the theory by Cope 

 and Osborn. 



(1) In the more advanced Triconodonts the upper and lower 

 molar crowns are shaped like reversed triangles shearing past 

 each other. In the upper jaw the triangular <three-cusped 

 crown is called the trigon; in the mandible the trigonid. In 

 essence the Tritubercular Theory calls for the derivation of 

 modern mammalian molar teeth from similar reversed triangles 

 and this part of the theory is still held to be correct. In the 

 upper molar the large internal cusp forming the apex of the 

 trigon is called the protocone. In the lower molar the outer 

 cusp which at first was thought to be homologous with this 

 since it also forms the apex of the triangle is the protoconid. 

 The paracone and metacone, the former in front and the latter 

 behind, lie at the basal angles of the upper molar triangle and 

 the analogous paraconid and metaconid occupy corresponding 

 positions on the lower molar crown. The suffix id was given 

 to the cusps of the mandibular teeth to distinguish them from 

 the supposedly homologous cusps of the upper teeth. It is 

 now recognized however that the protoconid is the homologue 

 of the paracone and not of the protocone. 



(2) According to the original enunciation of the Trituber- 

 cular Theory the para- and metacones or -conids primitively 

 lay in a straight line with the main cusp but later in phytogeny 

 their positions relative to this cusp alter and thus the trian- 

 gular crown was formed. It may possibly be that such a mode 

 of evolution did occur in the molar teeth of the Jurassic Tri- 



