420 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



antemolars, instead of explaining their exceptional number as due to 

 reduplication. This is the same dental formula as in the Didelphyidae. 

 It is occasionally observable in Ccenolestes, as noted by Bensley (1903, p. 

 124, PI. 5, ng. 38). 



The proximal half of the median incisor is preserved. As in Ccenolestes, 

 the enamel layer is confined to the outer side of the crown. So far as 

 can be judged from the part preserved, this tooth was of much the same 

 shape as in Ccenolestes. Following the enlarged incisor are five minute 

 teeth closely crowded and more or less pronate, which are interpreted as 

 three incisors, a canine and the anterior premolar. The anterior two are 

 represented by alveoli. The first alveolus is displaced toward the inner 

 side, lying beside instead of in front of the third incisor. The lateral in- 

 cisor, the canine and the anterior premolar are identical in shape. The 

 median and posterior premolars are double-rooted functional teeth. Both 

 carry large heels, that on the median premolar being much larger than 



FIG. 7. 



Halmarhiphtis nanus, right ramus, crown view, x f (No. 9593 American Museum of Natural 

 History). 



that on the posterior tooth. The crowns are laterally compressed, with the 

 principal cusp high and recurved. An anterior accessory basal cuspule 

 of microscopic proportions is observable on the inner side of the crown. 

 The first molar is not differentiated as a sectorial and is slightly smaller 

 than the second. The third is narrower than the second, and the fourth 

 quite small. All display the tuberculo-sectorial pattern (text fig. 7). The 

 trigonid is narrow, with the cusps separated by sharp notches. The talonid 

 is broad, with the hypoconid and entoconid enlarged and the hypoconulid 

 small, but distinct. The cusps of the trigonid and talonid are elevated to 

 the same general level in the three anterior molars. In the fourth, the 

 heel is depressed. The protoconid is slightly higher than the metaconid 

 in the first molar, but of approximately the same elevation in the second, 

 third and fourth. A prominent external cingulum is present on all the 

 molars, as in Ccenolestes and Didelphys. 



