FORT UjSriON OF CRAZY MOUNTAIN FIELD, MONT. 215 



triangular. There is a small anterointernal cusp, resembling a rudi- 

 mentary paraconid rather than the anterior basal cuspule of Didymidis, 

 and there is a very slight and uncertain indication of a rudimentary 

 basal metaconid. The talonid is very short and wide and vaguely 

 cusped. There are no other cuspules or cingula. 



Ml has a large but, in comparison with Didymidis, low trigonid 

 elongate anteroposteriorly. The talonid is very small, with distinct 

 but not prominent hypoconid, hypoconulid, and entoconid developed 

 on its raised rim. Its well-developed basin opens internally, between 

 entoconid and metaconid base. M2 was evidently much reduced. 

 The talonid is small but is elongate and basined, with the three cusps 

 poorly differentiated. The specimen is broken immediately posterior 

 to this, but from the shape of the talonid of M2 it cannot have been 

 followed by another tooth. Dimensions are as follows: Length P3, 2.0; 

 width P3, 1.4; length P4, 2.9; width P4, 1.9; length Mi, 3.8; width 

 Ml, 2.3. 



There is a specimen, U.S.N.M. no. 9295, a left maxilla with P^, P^- 

 M^, and the alveolus of the canine, that is probably the upper jaw of 

 Ididopajjpus mustelinus. Its size is exactly right for occlusion with 

 the type, it has P^ much smaller than in Didymidis, harmonious with 

 the smaller P4 of the type, and it has M'~^ markedly shorter than in 

 Didymidis, harmonious with the shorter M2 and heel of Mj of the type. 

 The only feature suggesting distinction is the embrasure between 

 P^ and M\ the outer angle of which is much more acute than the outer 

 angle of the trigonid of Mi, which fits into it. This, however, does not 

 prevent normal occlusion, as it might at first sight appear to do. In 

 Didymidis, also, the embrasure angle is more acute than the occluding 

 trigonid angle. Occlusion is not strictly orthal but is oblique, m part 

 ectal (in fact nearly analogous to the triconodont occlusion but, unlike 

 triconodonts, interlocldng). At the close of the bite the external 

 trigonid angle is internal to the embrasure angle, and the trigonid fits 

 mto the wider internal part of the embrasure and does not coincide 

 wdth it. It cannot be proved that this upper jaw is of Ididopappus, 

 but it is highly probable, and it should not be assumed to be distinct. 



The generic distinction of the upper jaw is very marked. The 

 canine was relatively large, its alveolar wall swollen, much as in the 

 most advanced and quite unlike the primitive species of Didymidis. 

 P^ is present and a small sunple tooth probably with two roots but 

 with alveolar mouths confluent. (P^ is represented by one root, the 

 specimen being broken here.) P^ is much smaller than in Didymidis. 

 Its ectoloph is similar in form. The pronounced inner spur, worn 

 but probably not cuspidate, is median and has a separate and strongly 

 divergent root. The ectoloph of P* is also similar to Didymidis but 

 has a more decided notch in the external contour and is without a 

 cingulum. The protocone spur is slenderer, perhaps less definitely 



119212— 3T 15 



