184 Field Columbian Museum — Geology, Vol. i. 



sessed by no other forms they might be regarded as of more than 

 generic importance, but in point of fact, the dental formula is 

 the same as that of Eucastor Leidy* and the later described Sig- 

 mogomphius Merriam,f both of which have true castoroid affinities. 

 As to the second feature, the isolated enamel lacunae are inde- 

 pendently developed in several forms and are doubtless depend- 

 ent upon the amount of wear to which the tooth is subjected. 

 Therefore this group has not been generally recognized by later 

 writers, and the genus has been provisionally included under the 

 Castoridce. 



Nevertheless the Mylagaulida appear to be a unique, and, until 

 now, isolated, form. The one prominent feature is the unusual 

 development of the premolar to the exclusion of the posterior- 

 lying teeth. Associated with this is the great strength and 

 shortness of the mandible, the prominence and anterior position 

 of the masseteric ridge and the depth of the ramus from alveolus 

 to angle. These tell an unmistakable story: Unusual capacity 

 for crushing or grinding and the attendant specialization of the 

 premolar to perform the function laid upon it. Just as in the 

 Carnivora, the lower first molar, lying immediately anterior to the 

 insertion of the masseter muscles, has developed into the princi- 

 pal shearing tooth ; so in these forms, the last premolar, stand- 

 ing in the same relation, has fitted itself for a crushing imple- 

 ment which, in M. Monodon, has reached the highest degree of 

 specialization known to Rodentia. It would be most interesting 

 to know the food habits of these unique animals. Such a ma- 

 choire might well have fitted them for nut-cracking or the crush- 

 ing of hard-shelled seeds ; but the abraded, crowns of all the 

 molars, together with their hyposodont character of growth, 

 makes it probable that the great tooth with its enamel pits was 

 used largely as a grinding surface. 



PHYLOGENY OF THE MYLAGAULIDiE. 



Upon the problem of phylogeny the limited material thus far 

 known throws comparative little light. An ancestor of Mesogau- 

 lus would be expected to have a less strongly developed premo- 

 lar with at least one lateral enamel inflection, to show traces of a 

 third lower molar, and perhaps retain further evidences of molar 

 roots. These conditions are largely fulfilled in the proposed 

 genus Protogaulus hippodus. In this form the mandible has the 



♦American Naturalist, July, 1881, p. 586. 



tBulletin of the Department of Geology, Univ. of California, vol. 1, No. 13. 



