1884.] 527 (Cope, 
produced to their point of intersection with each other. Such point of 
intersection is above the second true molar in this species ; in AM. superbus 
and M. chelydra it is above the posterior part of the second premolar. The 
area in these species enclosed by the lines in question is half as long again 
as wide, instead: of wider than long by 18mm. This difference is partly 
caused by the greater prominence and flatness of the postorbital angle of 
the frontal bone in the Jf. macrostegus, and the more anterior direction 
of the orbits, which I may add have none of the tendency to superior 
direction seen in M. chelydra. The wide triangular area thus enclosed on 
its external sides by the orbit and anterior temporal ridges, is perfectly 
flat. Such an area can hardly be defined in. the other species, and the 
surface there is rounded and descending. The malar bone is deep, flat 
and.a little oblique outwards, and the rim of the orbit projects a little, 
giving it a slight concavity. The orbit is deeper than wide. The anterior 
part of the zygomatic process of the squamosal is not protuberant below 
the orbit, but gradually rises outwards posteriorly, attaining its greatest 
expansion opposite the middle of the zygomatic foramen ; above, its course 
is for a time parallel with the middle line of the skull. The form of the 
zygomatic arch is more like that.of I chelydra than any other species, 
but it is not so much.expanded, especially anteriorly. Its inferior and 
posterior surface is, however, widened, making an angle with the ex- 
ternal or marginal surface, which is in turn separated by an angle from 
the superior and anterior surface ; at the middle of the arch the superior 
surface has a width of 19mm., and the external a width of 23mm.. The 
posterior angle rises to the plane of the summit of the sagittal crest, and 
the apex, which is less than a right angle, stands above the. external 
base of the postglenoid process. The preglenoid border is not exactly at 
right angles with the middle line, but makes.a slight angle outwards and 
forwards. The long diameter of the zygomatic foramen is parallel with 
it. The ridge along the pariétosquamosal suture is insignificent. The 
supraoccipital region is very prominent, and as in the other species of this 
genus is narrowed below by the disappearance of the posterior temporal 
or exoccipital crests. They are continued downwards.and disappear, leav- 
ing a wide convex surface above the foramen magnum. This is separated 
by the usual lateral fossa from the posterior temporal angles. 
The codssified mastoid and paroccipital processes much contract the 
auricular fossa below, but do not close it. The latter is contracted at the 
base of its terminal part, and is distally slender. The otic bulla is. the 
smallest known in the genus, it is compressed and oval, and not produced 
beyond the postglenoid processes either forwards, backwards or down- 
wards, in this differing much from the MZ. montanus. . It is separated by 
wide and equal intervals from this process, the glenoid surface, and the 
basisphenoid. It sends a process backwards and inwards. to a sutural 
junction with the basioccipital bone. The tympanic bone is flat below, 
and is united with the posterior base of the squamosal by a flat expansion. 
The postglenoid process is robust, and has the height and thickness equal, 
PROC. AMER. PHILOS. 800. XXI. 116. 80, PRINTED JUNE 9, 1884, 
