BIG BADLANDS OF SOUTH DAKOTA. 



67 



I am unable to separate these four groups on the basis of the 

 lower dentition. For systematic and stratigraphic purposes they may 

 be conceived as species, although some might wish to term them sub- 

 species. It will be noted that the distinctions between them are based 

 primarily on structural differences in the upper posterior premolars 

 which become increasingly molariform, the change beginning with 

 the fourth and working forward, a situation occurring in many mam- 

 malian groups and always regarded as indicative of progressive evo- 

 lution. No intermediate stages between these four types of structure 

 in p* have been observed and, in the absence of blending, they are 

 probably to be regarded as distinct species, on the basis of constant 

 association of constant differences. Whether these slight differences 

 in dental structure were accompanied by sexual antipathy between 

 the various types is, of course, outside the realm both of palaeontology 

 and available data. It is possible that the four types were derived 

 from each other in the order mentioned, ancestor and descendant 

 continuing to exist contemporaneously for a time (see table showing 

 vertical range). Other assumptions are equally possible. 



In groups A, B and C there is a sequence of size variations, inter- 

 grading by small increments, so that size must be ruled out for pur- 

 poses of specific discrimination unless we are content to change the 

 name every few millimeters. 



Nomenclature and Synonymy. 



In specific nomenclature there exists some confusion which, I 

 believe, can now be eliminated. There can be no question that the 

 tooth structure described in A and illustrated in Fig. i is identical 



Fig. I. Hyracodon arcidens, No. 125 18 Princeton University Geological 

 Museum. Upper premolar-molar series of the left side, crown view, three 

 fourths the natural size. 



