Hatchi'.r : Dknti'iioxs ok TrrAXoTUKKUM. 2G1 



tical position, ready for eruption at the posterior end of the horizon- 

 tal ramus. In this manner ms..^ and .^ successively occupy similar 

 positions in the ascending ramus. In the present specimen the outer 

 surface of the maxillary and lower jaw have been removed, revealing 

 pms.^, :} and | in their respective positions. Of these teeth pms.^ 

 and f are the more advanced and in each instance they appear in the 

 act of replacing their respective milk molars. Pms.A are not nearly 

 so advanced as the two preceding teeth and appear far below and be- 

 tween the roots of the fourth milk molars, which teeth they will later 

 replace. A somewhat careful examination has failed to reveal any 

 trace of the permanent incisors, canines or first premolars. It is pre- 

 sumable that these teeth will all appear somewhat later, when this 

 region of the skull has been sufficiently elongated to accommodate 

 them. 



Mei'hod and Order ix the Appearaxce of the Deciduous Teeth 

 and ix their replacement by the permanent dentition. 



A careful study of the comparative degrees of vvear shown by the 

 deciduous teeth in Nos. ii6 and 117 of our collections shows that 

 they made their appearance in the following order, d.m.|, |, i, i, y, 

 d.i.f, \, d.c.i. The canines were just cutting the gums when d.ms.| 

 and f were ready to drop out. 



Of the permanent teeth the superior and inferior first molars were 

 the first to appear, followed a little later by pms.f and f. The next 

 to cut the gums would be ms. |, which would appear shortly after the 

 second and third premolars. Then, after a considerable interval, 

 pnis. i would replace deciduous molars \, and these would be shortly 

 followed, as the jaws increased in length, by molars f, while the per- 

 manent canines and incisors would be the last to appear, but in what 

 order is not determinable from the material at hand. 



.Ml the dental and cranial characters above mentioned are well 

 shown in the accompanying illustrations, which have been made from 

 drawings by Mr. Rudolph Weber. 



Xo. 116 has been freed from the matrix and very skillfully mounted 

 as a disarticulated skull by Mr. A. S. Coggeshall, a feat rarely possible 

 in fossil skulls, more especially when they are from so old a formation 

 as the r)ligocene. 



