MURID&—ARVICOLIN/—EVOTOMYS. 133 
mys, the molars have only two parallel roots apiece, one directly behind the 
other, and both broad; and the rooting is simply the closing-up of the ends 
of the roots from failure of the pulps that in other Arvicoline are supplied 
indefinitely, causing the roots to persist open. On the other hand, in Murine, 
the roots of the molars are distinct diverging prongs, closed from the first ; 
there are at least three such prongs (two external and one internal) in Ameri- 
can Hesperomys or Sigmodont Murines, and even more in the Old World Mus, 
each perforating the alveolus separately. In Evotomys, there are but two 
such perforations of the alveolus, and these even are almost confluent. 
From Arvicola, the next most signal difference of Evotomys is seen in 
the construction of the bony palate. In Arvicola (e. g. amphibius or riparius), 
the palate behind has a little pit, or fossa, on each side opposite the last molars, 
and the whole space between them is depressed; and this depression is fissured, 
or excavated, by the advance from behind of the inter-pterygoid cavity, which 
either ends at the palate with a single curve, or with a double curve from the 
development of a little azygos process on the middle line of the posterior 
margin of the palate. “Thus,” to use Baird’s words, ‘there is a step from 
the plane of the bony palate to the bottom of the fossa, and another thence 
to the base of the skull or body of the sphenoid”; and the sides of the palate 
behind run out continuously into the pterygoids. Now, in Evotomys, all this 
depressed or fossate part of the palate is done away with; the palate ends 
by an abrupt transverse edge, as a straight shelf, opposite the middle molar 
(or rather opposite the space between the middle and last molar), leaving the 
excavation of the base of the skull apparent in the whole space between the 
last molars, and breaking all connection with the pterygoids. This construe- 
tion of palate, so unusual in Arvicoline, is, however, again found, with no 
appreciable difference, in the Lemmings; but, singularly enough, the genus 
Synaptomys, which repeats Evotomys in external form and AMyodes in denti- 
tion, has the palate constructed as in Arvicola. The curious interrelation of 
Myodes, Synaptomys, and Evotomys is sufficiently interesting without consid- 
ering the murine affinities of the latter; but, while we are on the subject of 
the palate, we may here allude to some of its conditions in murine forms. In 
Mus decumanus, the palate behind has no step downward or depressed fossate 
part, as Arvicola has, ending as a straight, sharp, transverse shelf, as in 
Evotomys; but then it reaches as a continuous plane far behind the last molar, 
and runs directly into the pterygoids on either hand, the median excavation 
