62 AMERICAN MESOZOIC MAMMALIA 



U.S.N.M. No. 2727. Left lower jaw with C, P2-4 and M1-7, much worn. Fig'd (as 

 Dryolestes vorax) , Marsh 1887, PI. IX, figs. 3-4. 



The following less important specimens have also been studied and referred to 

 this species and may be listed for the convenience of possible future workers : 



Y.P.M. Nos. 10658, 10660, 10661, 13720, 13721, 13723, 13724, 13725, 13726. 



U.S.N.M. Nos. 2729, 2731, 2732. 



A.M.N.H. No. 3001. Fig'd (as Phascolestes vorax), Osborn 1907, fig. 31. 



Horizon and Locality. — Morrison formation, Quarry 9, Como Bluflf, Wyoming. 

 Diagnosis. — Length M4, 1.2-1.4 mm. Inner surface of paraconid smooth. Trigo- 

 nid strongly basined. 



This abundant species shows considerable variation, and it may be possible to 

 subdivide it at some later date. The molar size may vary as much as 1 5 per cent, the 

 length of the trigonid and bulk of the metaconid vary slightly, and the size of the pre- 

 molars relative to the molars also varies. These and a few other slight differences could 

 easily be due to wear, to crushing, or to individual or sexual differences. 



Dentition 



The anterior incisor is large and procumbent, the following progressively smaller 

 and less procumbent, and they form an almost straight anteroposterior series. Each has 

 a single stout cylindrical root. The tip of each is spatulate, rounded, convex externally 

 and excavated internally above the basal swelling. 



The canine is a moderately stout, somewhat procumbent tooth implanted by two 

 quite distinct roots, the posterior a little more oblique and larger. The anterior profile 

 of the crown is strongly convex, the posterior somewhat concave. The external face is 

 convex, the internal excavated above the slight cingulum, which runs forward from the 

 small heel and merges with the anterior border. 



The premolars resemble the canine rather closely in morphology and are also like 

 those of Dryolestes save that there is no anterior accessory cusp and the cingulum is 

 continuous, running forward internally from the small heel to curve into the anterior 

 border. Pi is small, the following premolars progressively larger until P4 is of about 

 the same size as the canine. As in all dryolestids the change from the tall trenchant P4 

 to the lower tuberculosectorial Mi is very abrupt. There is no trace of the supposedly 

 gradual transition which these mammals have been supposed to show. They are farther 

 from a homodont condition than most later mammals. 



The eight molars are of nearly equal size, although they decrease slightly both 

 ways from Mo. Each has the same elements in nearly the same relationships as in 

 Dryolestes. The metaconid is remarkable. It does not rise to a bluntly pointed apex, as 

 in Dryolestes, but is columnar, usually somewhat expanded at the tip, and the apex, 

 when unworn, is bifid, divided into distinct anterior and posterior points. These are 

 separated by a small vertical furrow which runs from the apex down toward the trigo- 

 nid basin. This character is least marked on Mi and Ms but may be seen on all when 

 unworn. The talonid is of the same general character as in Dryolestes, but instead of 



