SACCOMYIDA—DIPODOMYIN Zi—DIPODOMYS. 525 
tinuous with the septum between the incisive foramina. That part of the 
palate constituted by the palatal bones is marked with several minute foramina. 
The palate ends behind with a sharp median spur; on either side of this is 
an emargination, and external to this a large fossa perforated with two fora- 
mina anteriorly, and a third and much larger one behind. Beyond the palatals 
themselves, the walls of the posterior nares are continued by the pterygoids, 
which are small and hamulate, the hook abutting against the petrosals. 
Between the ends of the pterygoids, and right across the middle line of the 
skull, the apices of the petrosals meet each other, forming a bridge beneath 
the basisphenoid. The posterior parts of the skull, behind those already 
considered, are almost. entirely occupied by the inflated elements of the tem- 
poral bone, between which lies the reduced basioccipital; this bone is nar- 
rowly acuminate, and is separated from the petrosals for its whole length by 
a continuous fissure, like that which, on the other side of the petrosals, sep- 
arates these bones from the alisphenoid and squamosal. The foramen magnum 
appears partly in this view, flanked by the slight condyles, outside of which 
are seen the small, distinct, flange like paroccipitals. 
The posterior view of the skull shows little but the inflated mastoids, 
with a cleft between, mostly occupied by the large foramen magnum, around 
which the contracted occipital bone appears as little more than the rim of this 
foramen. 
All the bones of the skull, as well as those of the top already so described, 
are thin and light; and the base of the cranium is remarkable for its exten- 
sive vacuities. Not only are the petrosals separated from their surrounding 
by great fissures on either side, rendering it almost necessary to their stability 
that they should abut against each other at their extremities; but there is also 
a singular separation of the alveolar portion of the maxillaries from the super- 
incumbent parts by a horizontal fissure, so that the palate appears as a sort 
of bridge between the fore and aft parts of the skull. Various peculiarities 
will appear more clearly from the following descriptions of individual bones. 
The complex temporal “bone” will be first considered, since the prime 
peculiarities of the skull result from the singular condition of hypertrophy 
under which several of the elements of this bone exist. ‘The topography of 
the parts.and the connections of the bone are curiously remodelled, as a con- 
sequence of the enormous inflation of the various otic elements and corre- 
sponding reduction of the squamosal. In the absence of investigations into 
