2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6o 



constituting the pair nearest the posterior end of the fragment are 

 separated in the median line by a distance of 9 mm., while the two 

 constituting the pair nearest the anterior one are separated by 4 mm. 

 As already mentioned, the space between the different pairs of 

 alveoli is occupied by a series of elliptical depressions of similar 

 form, but only a few millimeters deep. These depressions are 

 deepest at the middle of the series, the posterior ones being shallower 

 and longer, and the anterior ones shallower and shorter. 



The alveoli constituting the anterior pair, as already mentioned, 

 differ from the others in that they are divided by one or two rudi- 

 mentary transverse septa. These septa are convex backward and 

 emarginate above, so that they do not reach the level of the upper 

 surface of the jaw, except where they join the sides of the same and 

 the median longitudinal ridge. The anterior one is not visible on 

 the right half of the jaw, and on the left it is developed on the side 

 nearest the outer margin of the jaw. It is supposable that the teeth 

 belonging in these alveoli had a partially divided root, such as some- 

 times occurs in Plataiiista, Inia and other recent genera, but this 

 is by no means certain. The fragment in which they are situated 

 has been broken transversely into two pieces, and it is possible that 

 the two parts do not belong together. As the fracture is behind 

 the anterior pair of alveoli, however, this does not explain their 

 conformation, though it may be that the anterior piece belongs 

 farther forward, and that the mandible was a little expanded at the 

 extremity. 



The upper margins of the jaw are rounded, and the lateral bor- 

 ders of the median ridge are sinuous, owing to the fact that it ex- 

 pands opposite the points at which the pairs of alveoli and of ellipti- 

 cal depressions join. Along the center, it appears to have been as 

 high as the outer borders of the jaw and flat above. 



The sides of the jaw are convex and meet in a rounded angle in 

 the median line below. On each side are several rather large 

 foramina from which more or less distinct channels extend forward 

 toward the extremity of the jaw. There is, however, no continuous 

 longitudinal furrow such as is found in Stenedelphis, ScMzodelphis, 

 etc. 



The four teeth which accompany the mandible lack the greater 

 part of the root and .the crowns are worn in a peculiar manner, as 

 will be noted more in detail below. They seem too large for the 

 alveoli, but when placed in them it is observed that they fit quite 

 accurately and there is little doubt that they belong to the jaw already 

 described. 



