INTRODUCTION t 



denticles arose internally, forming a triangle, in the upper molars the 

 reptilian cone remained at the antero-cxternal angle and the two denticles 

 arose on the inner and posterior sides. 



It will thus be seen that the difference between the palseontological 

 and embryological theories is radical. The latter finds not only strong 

 support but a beautiful illustration by analogy in the normal modes 

 of evolution of the simple premolar teeth into the complex molai 

 type (see pages 194-200). It is developed in what may be termed the 

 ' PREMOLAR- ANALOGY ' theory. 



The accompanying figure clearly demonstrates the fundamental 

 difference between these theories. There is no middle ground between 

 them. If the premolar analogy be correct, the Osbornian cusp homo 

 logics of the upper teeth are of less value, but the homological iimm -//r/W///v 

 should be retained for convenience because it has found its way so largely 

 into literature. 



Although the writer has devoted a vast amount of time and study 

 to the development of the ' palseontological theory,' and is personally 

 inclined to believe in its tenability, there is no denying the great force 

 of the objections which have been urged against it and the need of 

 substantial proof from the early Mesozoic mammals, even in addition 

 to that which is freshly adduced in the present work. The arguments 

 pro and con are, therefore, partly stated in course of these collected essays, 

 and especially brought together in the new chapter IX. at the close. 



ORIGIN OF THE TRIANGLE. 



An important feature of the fourth principle is the position and mode 

 of origin of the cusps of the triangle. 



Four different modes of origin have been suggested : 



The first suggestion is that the triangle originated as such, the 

 denticles appearing from the outset on the inner and outer sides 

 respectively of the lower and upper cones. 



The second suggestion is that the denticles first arose on the anterior 

 and posterior sides of the reptilian cone, and were secondarily rotated 

 inwards and outwards respectively ; the latter is known as the ' cusp 

 rotation ' theory, or rather hypothesis. 



Osborn l set forth both the above modes guardedly in the following 

 language : 



i There can be no doubt that the cusps seen upon the inner face 

 of the inferior molars of this genus [Spcdacotherium] are homologous 

 with the para- and ineta-cones, and there are several facts which 



1 "The Structure and Classification of the Mesozoic Mammalia," .l>n\ A<-nd. Xt. Sci., 

 Phila., Vol. IX., No. 2, July, 1888, p. 243. 



