FORT UNIOX OF CRAZY MOUNTAUST FIELD, MONT. 203 



The individual dimensions of the type lower jaw are as follows: 

 Length P4, 5.0; width P4, 2.9; length Mi, 5.5; width Mi, 4.1; length 

 M2, 5.7; width M2, 5.2; length M3, 6.5; width Mg, 4.0. 



Genus SPANOXYODON Simpson 



Spanoxyodon Simpson, 1935d, p. 236. 



Type. — Spanoxyodon latrunculus Simpson. 



Distribution. — Middle Paleocene, Fort Union, Montana. 



Diagnosis. — Pi_2 absent and long diastema between canine and P3. 

 P3_4 much as in Chriacus, but P4 with metaconid larger, higher, and 

 more distiuct. Mi_2 about as in Chriacus or with trigonids slightly 

 lower and paraconids slightly less internal. 



Remarks. — Only one specimen referable to this genus is yet known, 

 and its characters are adequately summed up in the diagnosis. 



SPANOXYODON LATRUNCULUS Simpson 



Figure 53 

 Spanoxyodon latrunculus Simpson, 1935d, p. 236. 



Type. — U.S.N.M. no. 9287, left lower jaw with canine alveolus and 

 P3-M2. Collected by Dr. J. W. Gidley. 



Horizon and locality. — Gidley Quarry, Fort Union, Middle Paleocene 

 horizon. Crazy Mountain Field, Mont. 



Diagnosis. — Sole loiown species of genus. Measurements of type 

 as follows: Length P3, 3.8; width P3, 2.3; length P4, 5.0; width P4, 2.8; 

 length Ml, 5.2; width Mi, 3.9; length Mg, 5.8; width M2, 4.5. 



MIMOTRICENTES.sfl new genus 



Type. — Tricentes latidens Gidley. 



Distribution. — Middle Paleocene, Fort Union, Montana. 



Diagnosis. — Adaptively and structurally closely similar to Tri- 

 centes, but Pi present, no marked diastema, and molar paraconids 

 higher on crown and internal, not median, in position. 



Discussion. — In my preliminary paper I left the type of this genus 

 in Tricentes, where Dr. Gidley had placed it. On further study, how- 

 ever, I am forced to erect a new genus for it. The resemblance to 

 Tricentes is close, but in the latter Pi is apparently invariably absent 

 (the name Tricentes refers to the presence of only three premolars, 

 striking in this primitive fauna), and the paraconids of M2-3 are quite 

 different. The former might be only a primitive character and not 

 surely of generic rank if unaccompanied by other differences. The 

 character of the paraconid, however, as reduced as in Tricentes but 

 in a different way, removes this form from the Tricentes lineage and 

 may even mean that the relationship is not closer to that genus than 



•1 MiMoti an imitator+ Tricentes. 



