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FIELD COLUMBIAN MusEUM—GEOLOGY, VOL. 1. 
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 Hucastor Leidy* and the later described S7g- 
mogomphius Merriam,+ both of which have true castorozd affinities. 
As to the second feature, the isolated enamel lacunz 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 
Castoride. 
Nevertheless the W/y/agau/ide 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 AZ. 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 MYLAGAULIDA. 
Upon the problem of phylogeny the limited material thus far 
known throws comparative little light. An ancestor of Mesogau- 
dus 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. 
+Bulletin of the Department of Geology, Univ. of California, vol. 1, No. 13. 
