1884.] OJii [Cope. 



produced to their point of intersection with each other. Such point of 

 intersection is above the second true molar in this species ; in M. 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 t8mm. This difference is partly 

 caused by the greater prominence and flatness of the postorbital angle of 

 the frontal bone in the M. 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 M. 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 parietosquamosal 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 coossifled 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 M. 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, 



