TRITUBERCULY IN rxci'LATKS 69 



These differential features, it will be observed, follow the progressive 

 order of evolution in the molar crowns, for in ' modernization ' we see, 

 first, a degeneration of one or more of the primary cusps, then a remoddl///'/ 

 of the form of each cusp which may affect the twelve upper and lower 

 cusps very differently : for example, in such an ancient type as Mcni*- 

 cotherium we find one bunoid, two lophoid, and three selenoid cusps in 

 each of the upper molars. Third, the cusps begin to shift their positions 

 upon the crown. Fourth, they begin to develop unequally. Fifth, the 

 cingulum, which is primitively a complete peripheral band, begins to 

 disappear at certain points. Sixth, one or more peripheral cusps grow 

 up from the cingulum or upon the sides of the main cusps. Finally, 

 as the crests develop, the unequal development of the cusps causes the 

 transverse crests to unite at different points with the external crest. 



We find that if such analysis be applied to the elements of the molar 

 teeth we derive an absolutely infallible means of distinguishing different 

 lines of descent, for the above are the main features of divergent evolution. 



The primitive horse, tapir, rhinoceros and titanothere all stand apart 

 and cannot be confused ; each have their clear differentia. To check 

 the possibility of being misled by parallelism in molar form, we should 

 next observe the dental series as a whole, the proportionate development 

 of different members of the series the metatropJiism ; this often furnishes 

 the final proof or disproof of relationship, so far at least as can be derived 

 from the dentition alone. 



The above method of analysis is the outgrowth of an extremely 

 careful study and comparison of all the early Coudylarthra and Perisso- 

 dactyla, and it has been found necessary to exercise the closest scrutiny 

 to distinguish these early stages of divergence,* 



Now to turn to the subject of nomenclature, the system of terms was 

 originally based upon the actual homologies of the primary elements of 

 the trigon and trigonid, but in extending it to the other parts of the 

 crown and to the secondary cusps it was found that we must apply 

 similar terms to some of the later elements in the upper and lower teeth, 

 which are merely analogous to each other (performing a similar function) 

 occupying a similar position, and developing at about the same period, 

 otherwise the terms soon multiply so as to become a burden rather than 

 a convenience. [See Figs. 43, 135.] 



As far as possible, therefore, the same prefixes are retained for the 

 secondary parts of the molars as for the primary; thus the anterior 

 transverse crest of the upper molars is called the protoloph, as it is 

 invariably developed by the union of the protocone, protocouule and 

 paracone or parastyle, never from the metacone. The anterior transverse 



[*It should be noted that the molars of the Lower Eocene ancestors of the horse 

 (Eohippus) and of the tapir (Syttemodon) are closely similar. ED.] 



