Marsh Collection, Peabody Museum. 45 



remarkable part played by the tritubercular molar has been 

 unfolded by the discoveries and writings of Cope. It is 

 undoubtedly the ancestral molar type of the Primates, the 

 Carnivora, the ITngulata, the Cheiroptera, the Inseetivora, and 

 of several, if not all, of the Marsupialia. For example, we 

 can trace back the quadritubercular bnnodont, or parent ungu- 

 late type, to the tritubercular • this to the type with three 

 cones inline, which we may call the triconodont type, and this 

 in turn to the haplodont reptilian crown. A nomenclature may 

 be suggested for these cones, with reference to their order of 

 development and primitive position, to keep clearly before the 

 mind their homologies during secondary changes of form and 

 position. The primitive cone may be called the protocone ; 

 upon the anterior and posterior slopes of which appear, respec- 

 tively, the paracone and metacone. After the tritubercular 

 crown is produced by the rotation of the lateral cones, inwards 

 in the lower jaw and outwards in the upper yaw* the hypo- 

 cone, or heel, is developed, giving us the tubercular-sectorial 

 molar." We thus have the statements clearly and distinctly set 

 forth, (1) that upon the single cone there were at hrst devel- 

 oped cusps upon its anterior and posterior slopes giving a 

 linear arrangement, the " triconodont stage"; and (2) that 

 these anterior and posterior cusps rotated outward in the upper 

 jaw, leaving the main, primitive, or principal cusp, the proto- 

 cone, at the internal or lingual apex of the triangle, the " tri- 

 tubercular stage." The primitive element or that which 

 answers to the single conical cusp of the simple premolar is 

 thus supposed to be homologized and located in this type of 

 molar crown. For want of a more appropriate term I will 

 designate this view as the Theory of Cusp Rotation or Migra- 

 tion. 



As far as I am able to discover, the only direct paleontolog- 

 ical evidence in favor of such an explanation is to be found 

 in the lower molars of Spalacotherium, in which there is a 

 certain appearance of this rotation of the cusps, as assumed by 

 Osborn, but, as far as I know there are no molars of the upper 

 jaw which furnish any support whatever to such a view. 

 Aside from this almost total lack of evidence in its support, 

 the objections to this explanation are of such an insuperable 

 character that it is scarcely worthy of serious consideration. 

 In the first place, it appears so inherently improbable that in 

 the matter of cusp development, the premolars have had one 

 history and the molars another, that the evidence would require 

 to be of the most direct and positive kind even to place such 

 a proposition on the ground of reasonable probability. 



The order and manner of cusp addition, as pointed out by 

 Scott in the premolars, are grounded upon the most indisputa- 

 ble facts, and hence are entirely removed from the realm of 



* The italics in this last sentence are mine. 



