396 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 48. 



Description. — Dental formula: % ~y c \y V m ^ dpj, m-. j aw re j a _ 



tively long and slender; length of tooth series, including canine and 

 m 3 , 20 mm.; depth of jaw at c, 2.5 mm.; at m 2 , 3.7 mm. P 1 single 

 rooted but compressed laterally and with small posterior heel; p 2 

 and p 3 with anterior cusp budding off from the main cusp and a poste- 

 rior basal cusp (p 3 is somewhat larger and has slightly more promi- 

 nent accessory cusp than p 2 ); dp^ completely molariform with the 

 metaconid and protoconid subequal in size, but otherwise differ- 

 ing from the true molars in the somewhat narrower and smaller 

 heel and in the position and relatively larger size of the paraconid 

 which is a well-developed, anteriorly directed cusp quite distinct 

 from the metaconid. All the molars have low, basin-like heels with 

 relatively high inner cusps and high trigonids in which the meta- 

 conids and paraconids are closely appressed, forming a column which 

 exceeds the protoconid in height and equals it in bulk. The para- 

 conid, though close to the metaconid, is quite distinct at the summit. 

 The entoconid is an irregularly rounded and pointed cusp showing 

 on the inner side a tendency to split into two cuspules. 



Possible affinities and comparison with Myrmecobius. — The little 

 Fort Union mammal jaw described above presents some interesting 

 features. While these can not be interpreted with absolute finality, 

 or with more than a limited degree of certainty, because of the great 

 time interval between the Paleocene and the present day, and the 

 absence of known intermediate forms, the specimen nevertheless 

 strongly suggests relationship to the Myrmecobidae, as now repre- 

 sented by the single living species Myrmecobius fasciatus. Making 

 due allowance for its more primitive condition, this lower jaw, aside 

 from its fewer teeth, resembles that of Myrmecobius in a marked 

 degree. The special points of resemblance (see plate 23) are these : ( 1) 

 The jaw is elongated anterior to the four molariform teeth and has 

 a well-marked but short diastema between each of the premolars, 

 the first premolar and the canine, and the canine and last incisor. 

 The jaw is broken at the latter point and none of the incisors is pre- 

 served, but enough remains to show (2) that the jaw extended straight 

 forward from the canine and probably earned spaced incisors as in 

 Myrmecobius. (3) The canine is laterally compressed and semi- 

 premolariform, and the manner in which it is set into the jaw also 

 resembles a premolar. ( 4) Premolars 2 and 3 are narrow transversely 

 and long at the base, with well-developed anterior cusps budding out 

 from the main cusps, high above their bases, as in Myrmecobius] p 1 

 is small and single rooted, though laterally compressed like the 

 others. ( 5) All the teeth are entirely without cingula. ( 6) The inner 

 main cusps (metaconid and entoconid) of the molars are apparently 

 developing toward conate forms, while they equal or exceed the two 



