TK1TUBERCULAR EVOLUTION IN .MAMMALS :59 



simple tritubercular molars, of earlier periods. It follows that tritu- 

 berculy was an important factor in survival. 



(3) The definite homologies of the primary and to some decree 

 of the secondary cusps in the upper and lower molars can be 

 established. 



(4) The mode of succession of tooth forms favours the kinetogenesis 

 theory advanced by Ryder and Cope. 



There are three general observations to be made : 



First. In attempting to complete the history of each of the cusps 

 we naturally find that the paleeontological record is not sufficiently 

 perfect to admit of our following a certain type along a single phylum 

 back to the primitive type. We must at the outset proceed upon 

 the principle of similar effects, similar causes. For example, since 

 the history of the development of the intermediate tubercles- in the 

 superior molars of the Lemumidea (Pseudolemuroidea, Schlosser) is 

 perfectly clear during the Wasatch and Bridger epochs it is safe 

 to infer that the intermediate tubercles of the Ungulate molars, which 

 are fully developed in the underlying I'uerco, had the same history. 

 Second. There are in each period aberrant ////><> which embrace either 

 incomplete or degenerate tritubercular stages, i.e. a high specialization 

 in which the past record is obliterated, or, finally, stages in non- 

 tritubercular lines of development. Thin/. In the parallel evolution 

 of trituberculy in different phyla we find that the progression is by 

 no means uniform. In every geological period in which the fauna 

 is well known we observe progressive genera which outstrip the others 

 in reaching a certain stage of molar development, contrasted with 

 jx-i-xixft'iif types which represent arrested lower stages of development, 

 while between them are the central types which represent the degree 

 of evolution attained by the majority of genera. The latter may be 

 said to constitute the stage which is characteristic of the period. 



Tin xt(itjt-s of Irihini't'i-nli/ may now be defined as seen in different 

 types in their order of succession : 



I. Haplodont Type (Cope). 1 A simple conical crown. The fan^ 

 usually single and not distinguished from the crown. This type has 

 not as yet been discovered among the primitive Mammalia. 



A. Protodoni Sub-Type. 2 The crown with one main cone, and 

 lateral accessory cuspules ; the fang grooved. There is some cpiiestion 

 as to the advantage of distinguishing this as a type, for it stands 



1 " The Homologies and Origin of the types of Molar Teeth in the Mammalia 

 Educabilia," Journ. Phila. Acad., 1874. The term Homodont was previously applinl 

 to this type by Riitimeyer, " Odontographie der Hufthiere, etc," }'> rhr. d. Xalnrl'or--<-h. 

 Gesellsch. in Basel, Band III., 1S63, p. 563. ]ii the writer's opinion this term has 

 acquired a special significance as applied to a whole series of teeth, viz., the reverse 

 of " heterodont," and may well be retained in this sense. 



2 Osborn, op. tit. , p. 222. 



