128 THE MAMMALIA OF THE DEEP EIVER BEDS. 



MALE. FEMALE. 



M, M. 



Lower second premolar, length 013 .013 



Lower third premolar, length 013 .014 



Lower fourth premolar, length 015 .014 



Lower molar series, length ! 059 .064 



Lower first molar, length 014 .014 



Lower second molar, length 017 .018 



Lower third molar, length 028 .032 



The Milk Dentition is like that of the older genera in that cus, which in Mery- 

 chyus is molariform and composed of four crescents, is like neither molar nor pre- 

 molar. The anterior half of the crown is a compressed protocone with trenchant 

 edges, like the corresponding cusp in the premolars, but thicker transversely, and 

 with a fossette upon its anterior face. Except for its greater thickness, this portion 

 of the crown is like the whole of cuj or the corresponding premolar. The posterior 

 part of the crown is composed of a pair of transversely placed crescents, the trito- 

 and tetartocones, and resembles half of a molar. As I have elsewhere pointed out, 

 with reference to the more ancient members of the family, this tooth plainly shows 

 that in the upper milk molars the homologies of the cusps, as determined by their 

 position, are the same as in the premolars, but the order in which these cusps appear 

 is altogether peculiar, being as follows : proto-, trito-, tetarto- and deuterocones (ISTo. 

 29, p. 441). In the lower series, S7a and O are like their successors in the permanent 

 dentition, while cT74 is of the usual artiodactyl pattern, consisting of three pairs of 

 crescents ; of these, the anterior pair is formed by the paraconid and an element 

 internal to it, to which, as it occurs only among the artiodactyls, I have not thought 

 it worth while to give a special name. I cannot determine whether pj. has a predc-" 

 cessor in the milk series, as it has in Oreodon, though there is some reason to think 

 that this is the case. If so, the change takes place at an early period, before any of 

 the other milk molars are shed. The milk canines and incisors differ from the per- 

 manent ones merely in size. 



In short, the temporary dentition of Mesoreodon departs from that of Merycliyus 

 more widely than does the permanent one, though in this connection it should be 

 remembered that the temporary teeth are not known in the earlier species of this 

 genus, M. zygomaticus and M. imriogonus, and as these species have a permanent 

 dentition which in one or the other respect recalls that of the older genera, it may 

 well happen that the temporary dentition of these sjjecies will also prove to be inter- 

 mediate between that of the typical Merycliyus species from the upper Loup Fork 

 and that of the more ancient foims of the family. 



Tlie Skull (PL IY, Fig. 32; PL V, Fig. 35). The structure of the skull is so 



