134 AMERICAN MESOZOIC MAMMALIA 



Type 6 



Trigonid and talonid nearly equal in height. Pr'' 

 higher than me"^ and me<' than pa"^. Anterior cingulum ab- 

 sent. 



The absence of the anterior cingulum is remarkable in 

 this fauna. 



Type 7 

 Trigonid somewhat higher than talonid. Pr-^ slightly 

 Fig. 52. Lance pediomyine. higher than me<*. Pa-^ much reduced. Talonid cusps unusu- 

 Fragment of left lower jaw ^j, distinct from each other and sharp, 

 with two premolars. Y.P.M. •' ' 



No. 10693. Five times natu- Type 8 



T3.1 S1Z6 



Trigonid little higher than talonid, compressed trans- 

 versely, much longer than broad. Pv^ higher than me'' and latter higher than pa"*. 



This is the type of lower molar associated by Osborn with his Synconodon (see 

 below). It is somewhat doubtful to what extent the essential character, the lateral com- 

 pression, is of taxonomic value and whether it is not rather due to position in the series. 

 The teeth have all the characters of the milk molars of modern opossums. 



Type 9 

 Trigonid higher than talonid. Yx^ and me-^ about equal. Pa'' almost vestigial. Two 

 very distinct talonid cusps. 



C. DOUBTFUL OR INVALID FORMS 



It is convenient to gather here several supposed Cretaceous mammals, whether 

 based on upper or lower teeth, the status of which is dubious. 



Boreodon Lambe 

 1902. Boreodon, Lambe, Con. Can. Pale., IH, 79. 



Species. — B. matutinus Lambe, genotype, loc. cit. Type : a single premolar from 

 the Belly River, Red Deer River, Alberta. 



Description. — "The crown is in the form of a laterally compressed cone, some- 

 what rounded above, angular in front, evenly rounded behind, with a well defined, 

 undulating cingulum encircling its base. A slight lateral concavity, more clearly de- 

 fined within, occurs on each side of the tooth near the base of the crown in line with the 

 cleft between the roots. The cingulum is angular and prominent at either end, becom- 

 ing rounded and broader on the sides." 



This form was referred to the Multituberculata by Lambe but this was certainly 

 incorrect, as Matthew (1916) has remarked. It is probably the premolar of a didel- 

 phid, but is indeterminate. 



