` ? 3 
56 The Extinct Rodentia of North America. (January, 
‘The Eucastor tortus was larger than any of the extinct species 
_ of Castor, but was considerably smaller than the beaver. The 
Diobroticus trogontherium of Europe was a still larger species, one- 
fifth larger than the beaver in dimensions. 
MYLAGAULUs Cope. Be 
The reduction of the posterior molars, seen in Eucastor, is — 
carried to a still higher degree in — 
this genus. The last or fourth mo- — 
lar has disappeared, and the indica- ` 
tions from the specimens are, that — 
the third was early shed. The sec- — 
ond is a small tooth, while the first — 
“is enormous, and performed the — 
greater part of the function of mas- ‘ 
tication. : 
The characters of the genus are: | 
Inferior molars three, rootless; the — 
first much larger than the others. — 
Enamel inclosing the first molar not ~ 
` inflected ; but numerous fossettes on — 
Fic. 12.—a, b,c, Mylagaulus mon 
odon Cope, lower j jaw and a separate 
tooth, natural size; d, M. sesquipe- the grinding surface of the crown, — 
dalis, first inferior molar, nat. si 
Original, From the Loup Fork epoch whose long diameter is anteroposte- 
of Nebraska. rior. 4 
The only lower jaw ofa species of this genus in my pòsses 4 
presents a small part of the base of the angle and of the coronoid — 
‘process. These parts are so nearly in the plane of the incisive 
alveolus as to lead to the belief that the genus Mylagaulus 
longs to the sub-order Sciuromorpha. The rootless teeth with 
deep enamel fossettes approximates it to the Castoridz, but it 
pears to me that a new family group must be established for its 
reception. Such characters are the presence of only three in- 
of the external sheathing enamel. 
-whether the Hystrix refossa Gerv, has any relation to this famil 
Two species of this genus are known: a larger M. monodi 
Cope, and a smaller, M. sesquipedalis Cope. Both are from the 
Loup Fork epoch of Nebraska. The former was about the size 
of the wood-chuck (Arctomys monax), to judge by the dimensions i 
of its lower jaw. Itis larger than the M. sesquipedalis, and has 
different arrangement of the enamel fossettes. In that species, i 
pes 
