MAY, 1921. AMERICAN MARSUPIAL, C^ENOLESTES OSGOOD. 127 



lapped by the anterior shelf of the first molar. Thus, although higher, 

 it is slightly shorter than the middle premolar. On its antero-external 

 surface is a faint indication of a cingulum. 



Discussion. The lower premolars, like the upper ones, show tenden- 

 cies toward the condition prevalent in diprotodonts. This is evidenced 

 principally by the reduction of the anterior teeth and the separation of 

 the posterior one from them to a position at the head of the row of 

 grinding teeth. The only approach to this among polyprotodonts is 

 seen in the Peramelidae. 



LOWER MOLARS. 



Description. The first lower molar has a relatively small trigonid 

 and an enlarged talonid. The protoconid, which is only very slightly 

 larger and higher than the other cusps of the trigonid, is connected by 

 crests with the paraconid and metaconid. The talonid is broad and 

 extended and the lateral angle between the protoconid and the entoconid 

 is relatively shallow. No hypoconulid is distinguishable. A distinct 

 external cingulum is present and continues forward to form a slight 

 anterior shelf. The second lower molar is similar in general to the first, 

 but is larger and its trigon is relatively wider, although still somewhat 

 narrower than the talonid. There is a slight indication of a hypoconulid 

 and the paraconid is flattened or even emarginate anteriorly where it 

 meets the entoconid of the preceding tooth. The third molar is smaller 

 than the second and approximately equal to the first. The talonid 

 exceeds the trigon in width only very slightly and the general outline of 

 the tooth from above is rectangular. The flattening of the paraconid is 

 more pronounced and almost divides it into two distinct tubercles. 



The fourth molar is compressed and greatly reduced in size, being 

 narrower and only slightly longer than the trigon of the preceding tooth. 

 The metaconid persists nearly as large and well-differentiated as in the 

 other molars but the other cusps are rudimentary. The entoconid is 

 slightly elevated and distinguishable; the hypoconid is represented 

 merely by the postero-internal angle of the short blunt talonid; the 

 protoconid has disappeared; and the imperfectly divided paraconid is 

 vestigial. Despite its small size, this tooth is two-rooted, agreeing in 

 this respect with the extinct form Haltnariphus, but disagreeing with 

 Garzonia. 



Discussion. The lower molars, as compared with those of primitive 

 polyprotodonts, show three conspicuous tendencies. 



(i) The reduction of the protoconid and the general reduction in 

 size and height of the outer cusps as compared to theinnerones. (2) The 

 elevation of crests connecting all the cusps. (3) The production of a 



