324 PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 



irregular tube ; it is wide in the parietal zone, contracting gently to the 

 postorbital constriction, thence gradually expanding again to the fore- 

 head. No sagittal crest is present, merely a broad and obscurely marked 

 sagittal area. The preorbital region, or rostrum, is unusually long and 

 has nearly straight, parallel sides, though narrowing forward slightly ; the 

 preorbital fossae are very shallow and hardly visible from above. 



The occiput is relatively high and has a distinct forward inclination ; the 

 crest would appear not to have been prominent, though the loss of the 

 supraoccipital leaves this somewhat uncertain ; the foramen magnum is 

 transversely elliptical. The basioccipital is long and broad, strongly 

 convex behind, almost flat in front ; from the foramen magnum a shallow 

 median groove is continued for a short distance, a feature which has not 

 been observed in any other species ; the condylar foramen is small and 

 the foramen lacerum posterius is large. The low and wide exoccipitals do 

 not meet in the median line, leaving between them a narrow space where 

 the supraoccipital reaches the foramen magnum ; above the foramen the 

 exoccipitals are produced almost as far as the hinder margin of the con- 

 dyles, which are rather small ; the paroccipital processes are obsolete. 

 The supraoccipital is lacking, but its shape is sufficiently indicated by the 

 surrounding sutures ; it was very broad and shield-shaped, with a median 

 ventral process, which, as already mentioned, extended between the exoc- 

 cipitals to the foramen magnum ; it must have formed much the greater 

 portion of the occipital surface, and dorsally was reflected forward for a 

 short distance between the divergent parietals. The exposed portion of 

 the periotic is a narrow vertical ridge, with a groove on each side of it, 

 and ends ventrally in a roughened process. The tympanics have been 

 lost, but the size of the fossae thus exposed apparently indicates that these 

 bones were larger than in the other genera. 



In marked contrast to the smaller Gravigrada of the period, the parie- 

 tals are, at least along the hinder border, very thick and filled with can- 

 celli and small sinuses ; over most of their area these bones have a 

 smooth, dense surface, and anteriorly they have short median processes 

 between the frontals. So far as it is not covered by the pterygoids, the 

 basisphenoid is broad and nearly plane, with a large tubercle at each pos- 

 tero-internal angle, just behind the pterygoid suture. The presphenoid is 

 concealed by the vomer, the median plate of which descends more gradu- 

 ally and is not so conspicuous in the posterior nares as it is in P. longi- 



