62 SINCLAIR— THE SMALL ENTELODONTS 



of molar pattern and premolar spacing in association with skull struc- 

 tures like those of Mr. Troxell's clavus and darhyi, which, in turn, 

 are not essentially unlike those of A. mortoni. An examination of 

 Mr. Troxell's text and Fig. 3^" will show that m^ of ^. clavus claznis 

 is of much the same general type which I have attempted to represent 

 in Fig. 3 D. Mr. Troxell describes it as follows : " The protocone 

 has a position midway fore and aft, and has encroached on the hypo- 

 cone so that the latter is scarcely visible ; it is, in fact, smaller than 

 the metaconule and is nothing more than a heel, continuous with the 

 hypostyle. The metacone is also lower than the metaconule, which 

 already shows a lake of dentine by reason of the wear." In the in- 

 ferior dentition, in m, " the hypoconid is strongly developed ; on the 

 Other hand, the entoconid is weak. A distinct posterior heel may be 

 seen. Mg is much like m,, except that a very marked posterior heel 

 and a less strong hypoconid are observed," and these points are well 

 brought out in the drawings accompanying Mr. Troxell's text. In 



Fig. 5. Archceotherium mortoni, No. 9315 American Museum. Right 

 upper m3 showing an extreme degree of cusp reduction. One half the natural 

 size. 



A. clavus darhyi m^ is " round or slightly oval, due to the strong lobes 

 of the para- and hypocones. There is a moderate cingulum anterior 

 only and no heel. The protocone is forward of the mid-line. The 

 hypocone is strong." In Fig. 8 of Mr. Troxell's paper, where the 

 lower teeth, with somewhat worn crowns, are represented, there is a 

 weak entoconid on m^ ; no hypoconulid is shown on this tooth, while 

 on mg there is a very large hypoconid and a posterior and internal 

 ledge from which the remaining cusps do not seem well differentiated. 

 Perhaps this is what Mr. Troxell refers to when he writes of "the 

 absence of heel in mg " being an unusual feature which distinguishes 

 it " from the other specimens present." Neither of the Yale speci- 

 mens possess unspaced lower premolars. 



In the presence of this melange of characters I am quite unable 

 to separate species on the basis of constant association of constant 

 differences. Certain characters, like the complication of crown pat- 

 io Loc. cit., pp. 364, 365- 



