44 OREODON. 



Vk'c adopt a hypothesis which supposes the hxteral incisors of Oreodon to be trans- 

 formed canines, and the functional canines to be the transformed first of the 

 normal series of seven molars. ■ The latter view is favored by the absence in 

 Oreodon of the first of the normal number of molars, and also by the unusual 

 position of the inferior canine tooth. Further, the latter has almost the exact form 

 which would be produced by merely prolonging the crown of the first functional 

 premolar. On the other hand, in Palceotherium, the lower and upper canines have 

 the same relative position as in Oreodon, and yet the lower jaw has the normal 

 number of premolars. 



Temporary Dentition and Order of Succession. — The deciduous dentition of 

 Oreodon, so far as can be ascertained from the specimens under investigation, is 

 expressed by the following formula : — 



. ? 11 2 2 11 



I. _ c. p.m. m. 



? 11^ 2 2 11 



In the order of protrusion of the temporary molars, judging from the relative 

 extent of abrasion which these teeth have undergone in the specimens under 

 obsei'vation, the true ixiolar is first, and then follow the premolars in succession 

 from behind forward. 



Form of the Temporary Molars.— {V\. IV. Figs. 4, 5; V. 2, 3; VI. 6, 7, 10, 11.) 

 The superior temporary true molar has exactly the same form as the permanent 

 true molars, but is about one-sixth less in size than the first of these. (V. 2, 3.) 



The crown of the second upper premolar is composed of three lobes like those 

 of the true molars : two posterior and transverse, the other anterior and oj^posite 

 to them. It resembles very much the crown of the fourth permanent premolar in 

 conjunction with that of a small third premolar. The anterior lobe at its inner side 

 is connected with the adjacent side of the postero-internal lobe by means of a 

 shallow fold, which forms a cid-de-sac between the two. 



The anterior premolar has nearly the same form as the corresponding permanent 

 tooth, but is smaller in size. Its antero-internal cids-de-sac are not as deep as in 

 the latter, and the external of these is twice as broad as the other, but is shallower. 



The inferior deciduous true molar, as in all existing ruminants, possesses three 

 pairs of symmetrical lobes, which have the same form, and the same relative posi- 

 tion with one another as those of the permanent true molars, but which decrease 

 in size from behind forward. (IV. 4, 5; VI. 6, 7, 10, 11.) 



The two deciduous premolars of the lower jaw closely resemble in form the 

 corresponding permanent teeth. 



The normal first superior molar appears to belong to the permanent series, 

 succeeds all the temporary molars in the order of protrusion, and has no deciduous 

 predecessor. 



The permanent true molars successively protrude and occupy their functional 

 position before any of the deciduous molars are shed. The displacement of the 

 latter by their permanent successors, appears to begin with the eruption of the 

 last of these, which is followed by those in advance. The first permanent 

 premolar of the upper jaw appears to have protruded after the deciduous teeth, 



