HAPLODONTIDZ—SKULL OF HAPLODON RUFUS. 569 
The lower jaw, as Richardson originally observed, is altogether heavier 
than usual among Rodents ; and it is peculiar in the condition of the descend- 
ing process, which in this case is a broad, flat plate, so far twisted around that 
it is horizontal, and its back edge is a straight transverse line. When the 
two halves of the jaw are separated, each will stand alone upright upon the 
table, supported by this broad plate, which has twice the width of any other 
part of the jaw. When the two halves of the jaw are in situ, the distance 
across from tip to tip of these plates equals the distance from each of them 
‘to the ends of the incisors; so that the three extreme points of the whole 
jaw represent the angles of an equilateral triangle; while the inner corners 
of these plates are only separated by a distance equal to the molar interspace. 
From the back outer corner of this plate, which is knobbed, and represents 
the angle of the jaw proper, there sweeps up to the condyle with gentle 
concavity the edge of a thin plate, which trends strongly obliquely inward 
and forward as well as upward, so far is the angle of the jaw carried out from 
the general axis of the bone. The inner end of this plate rounds off to the 
body of the bone; it projects so far inward that it is separated from its fellow 
by a distance only equal to the intermolar space. The condylar process is 
rather low, rising upward vertically in one plane, but with strong backward 
obliquity (about 45°. from the plane of the molar crowns). The articular 
head is nodular, with some posterior prolongation, excepting which latter it 
is rather broader across than in the longitudinal direction. The coronoid 
process is very conspicuous, reaching far above the condyle; it is a thin 
vertical lamina, broadly falciform, with the apex, which is not as high as the 
convex edge anterior to it, twisted somewhat outward. The anterior border 
of this plate forms a letter S, very convex above where it curves almost semi- 
circularly over to the apex, less strongly concave below where it sinks into 
the body of the bone. Similarly, the emargination between the condyle and 
apex of the coronoid is nearly semicircular. The body of the jaw, on the 
outer side, opposite the middle of the molar series, shows a strong oblique ridge, 
indicating the limit of the muscular impression. There is a large foramen 
at the base of the condylar process on the inner side; the mental foramen 
is situated on the outer side, midway between the molars and the incisors. 
It is to be regretted that the maturity of all the specimens (5 in number) 
examined prevents recognition-of most of the individual bones of the skull, 
most of the sutures being already obliterated. The part taken by the malar 
