200 Annals of the Carnegie Museum. 



is also very much less drooping than in P. carrikcri, but in its general 

 construction it is otherwise quite similar and has a greater downward 

 thrust than in either P. chelydra or P. hollandi. As in the earlier 

 species the deep temporal fossa is surrounded by prominent borders 

 formed above and behind by the temporal, sagittal, lamboidal, and 

 posttemporal crests; formed below by the broadly expanded zygomatic 

 arch, and in front by the united postorbital processes of the frontal 

 and jugal. The external cranial wall has prominent ridges, which 

 extend forward and downward from the posterior portion of the 

 parietal to the lower part of the squamosal, serving for the attachment 

 of muscles in this region. The temporal muscle was evidently of 

 considerable size and thickness. The glenoid cavity is regularly 

 convex fore-and-aft, and bounded on the postero-internal angle by a 

 postglenoid process, which is thick antero-posteriorly, quite heavy 

 as in other species, and unlike the transversely broad process in 

 Merycoch(rnis. The paroccipital and postglenoid processes are slightly 

 closer together than in P. carrikeri, and in this respect more nearly 

 resemble those in P. chelydra and P. hollandi, in which they are still 

 closer together. This character suggests the conditions found in the 

 later forms Merycocha'rus Leidy, and Prononwtherium Douglass, in 

 which the postglenoid, the paroccipital, and the occipital condyle are 

 very close together. In proportion to the large tympanic bulla the 

 external auditory meatus is quite small, while in the older species it 

 is larger, though the bulla is smaller. 



The region of the sphenoid bones of this species does not appear to 

 be in any respect different from that of P. carrikeri, except the foramen 

 ovale, which is situated more internally in the type of the present form. 

 The pterygoids are apparently somewhat larger and the posterior 

 nares are further back than in P. carrikeri, which is also true of 

 P. chelydra and P. hollandi, another character which approaches 

 conditions in the later Miocene forms. 



The frontal region is more convex from side to side over the orbits 

 than in P. chelydra or P. hollandi and more like that in carrikeri, but 

 the posterior portion of the frontal does not take the upward turn to 

 meet the sudden rise of the sagittal crest as in the latter species nor 

 are the temporal ridges of such conspicuous prominence when the 

 skull is viewed from in front. The position of the orbit is approxi- 

 mately in the same position as in P. carrikeri and lower down on the side 

 of the face than in P. hollandi. The supratemporal foramen is small 



