SKCOXl) OUTLINK OF TKITUHKItf T I.Y 75 



for the writer to take the " primitive polybuny " hypothesis seriously, 

 although it is advocated more or less positively by such able inorpho- 

 logists'" as For syth- Major, f Lankester, Goodrich^ and Parker. The fact 

 that the Multituberculates and Monotrenies and certain Rodents 

 exhibiting this type are primitive is no evidence that the polybunic 

 type itself is primitive. We know nothing of the history of the 

 degenerate Monotreme teeth, but we know that the further we go 

 back among the ancestors of the Multituberculates and Eoclents the 

 less " polybunic " and more tritubercular they appear. 



This demonstration once made, as a matter of convenience in 

 thought and description, we must revise the old systems of nomen- 

 clature which were based upon secondary forms rather than upon 

 primary homologies, and which, as a rule, differ in every type of 

 mammals and among odontologists of every land and establish a new 

 odontography or descriptive method. Finally, we must trace out all 

 the lines of divergence in both forms and determine the principles 

 which guide them. The importance of a uniform nomenclature is seen 

 at once in the accompanying table of terms used among the rhinoceroses 

 and horses alone. It could not have been anticipated that the diverse 

 molars of the horse and of the rhinoceros, for example, would be limited 

 in their variations, in a late geological period, by their unity of origin in 

 an extremely early^ geological period. Yet such is undoubtedly the case. 

 Compare the accompanying figures of Merychippus (Fig. 162) and of 

 Acc/'xf/nrium (Figs. 175, 176). Imagine that you see the simple 

 bunodont molar of such a form as Owen's Hyracotherium rt'lpiceps 

 [l&porinum] (Fig. 159), underlying these diverse crests and crescents. 

 Consult Taeker's Zxr Kenntiri^ ff/r Odontogenese lei Unt/n/tt/cn and you 

 will find that this sexitubercular archetype is not imaginary, but is a 

 constantly recurring fact of embryonic development all the crests and 

 crescents being preceded in the embryo by simple cones. Then compare 

 carefully the variations in the two teeth as follows : The two " cement 

 lakes " of Merychippus with the two " fossettes " of Aceratherium, enclosed 

 in the former by crescentic spurs, and in the latter by the " antecrochet ' 

 and " crochet " ; the posterior " lake " and " fossette " similarly enclosed 

 by an upgrowth of the posterior basal cingulum. Can any one question 

 the homologies between these secondary adaptations to a diet of grasses 

 when it is seen that they spring from the same primary cusp centres ? 

 In the lower Eocene the sexitubercular prototype passes directly back 

 into the tritubercular archetype. So throughout the whole mammalian 

 scale not only ungulates, but primates, carnivores, insectivores, rodents are 



*[This list should have included especially Dr. Florentine Ameghino (see pp. 201-204 of 

 this work). H.F.O.]' t [See p. '205.] 



I [See p. 206.] [That is, relatively early, i.e.. the Cretaceous period. En.] 



