INTRODUCTION 5 



principle ; it is a process analogous to budding or outgrowth in other 

 tissues. 



An opposed theory, advanced by Ameghino, Rose, and Kukenthal is 

 that of concrescence, namely, that from an original large supply of conical 

 reptilian teeth in the primitive longer jaws, and from the multiple 

 succession or replacement of such teeth, the cones were clustered or 

 grouped, by fours or more, and thus arose respectively the quadri- 

 tubercular, and multitubercular or polybunic types, the tritubercular 

 and triconodont stages being secondary (Ameghino). 



IV. FOURTH PRINCIPLE. EEVERSED UPPER AND LOWER TRIANGLES. 



Still another or fourth principle, entirely distinct from the fore- 

 going, is involved in the sentence quoted above, " Third, the denticles to 

 the inner side of the crown [in the lower jaw] forming a three-sided 

 prism, with tritubercular apex, which alternates with that of the 

 opposite [or upper'] jaw." This principle involves the theory which rests 

 upon strong but perhaps not altogether conclusive pakeontological 

 evidence (pp. 32, 43, 217) that in the lower molars the reptilian cone, is 

 external and the two denticles internal, while in the upper molars the reverse 

 is the case, namely, the reptilian cone is internal and the denticles are 

 external. This principle, if a true one, enables us to establish a kind of 

 serial homology between the main primary cones and secondary denticles 

 or cusps of the upper and lower teeth respectively. Osborn expressed 

 such an homology in a system of nomenclature (protocone, paracone, 

 metacone, etc.), which Professor Cope welcomed and accepted. 



According to this principle, the evolution and relation of both the 

 upper and lower molars are those of a pair of reversed triangles in 

 every stage above the protodont and triconodont ; thus, it might be 

 simply known as the trigonal theory ; but since it was based by Cope 

 and the writer entirely upon the evidence afforded by the Mesozoic 

 molar teeth, it may be more strictly termed the ' PAL/EONTOLOGICAL 

 THEORY.' 



As applied to the upper molars, this theory and the homologies it 

 involves with the lower molars have been far more vigorously and 

 generally opposed than either of the other principles ; in fact, the chief 

 weight of opinion has now gathered against it from three different 

 classes of positive evidence, namely, embryological, anatomical, and 

 palfeontological by comparison with premolar evolution, also from the 

 negative argument that the evidence at hand among the Mesozoic 

 mammals does not demonstrate the principle in the upper teeth. 



The theory opposing the pakeontological theory in morphological 

 contrast first sprang from embryological evidence, and may, therefore, 



