1900.] STRUCTURE OF THE MUSK-OX. 711 



primitive type. It is consequently impossible to derive Ovibos 

 from the Sheep on account of this. 



In the youug summer calf the three milk-premolars of the upper 

 jaw are developed, and the first molar is just protruding from the 

 socket. The middle aud three hindmost premolars are bilobed, 

 and on the latter can be seen, between the two lobes on the inner 

 side, a small, but distinct, ridge representing the accessory column. 

 In the same calf the three mandibular milk-premolars are developed 

 and the first molar just protruding. The foremost deciduous pre- 

 molar is small and simple, the middle is two-lobed, and the hind- 

 most is three-lobed. In the domestic calf there are on the outer 

 side of this hindmost mandibular milk-premolar two well developed, 



Fig. 10. 



Section through a milk-incisor of a calf of Bos taitrus. 



although shorter, additional folds. These are not entirely absent 

 in the Musk-calf, but only developed as two short tubercles in a 

 corresponding situation at the base of the tooth. From Fcichardson's 

 description (7. c. p. 71) it becomes evident that the deciduous 

 molars are retained at least a year. 



The permanent premolars of the Musk-ox are one-lobed ; and 

 the molars are two-lobed except the third mandibular molar, which 

 is three-lobed, as is already recorded by previous authors. As 

 Boyd Davvkins says (I.e. p. 8), the upper true molars of the Musk- 

 ox "are differentiated from those of Bison and Buffalo by the 



