14 SCOTT. [Vol. V. 



in P. Wilsoni it is long, narrow, and rounded, being reduced to 

 a mere rod by the enormously inflated tympanic bullae, while in 

 P. labiatum the more moderate size of the bullae allows it to 

 expand into quite a broad plate, with well-defined keel and 

 lateral rugosities. The condyles are sessile, not projecting at 

 all behind the plane of the occiput ; inferiorly they meet in the 

 median line, and are not separated by a groove, as is the case in 

 the recent species. Though the articular surface is continued 

 well forward on the basi-occipital, there is not formed that 

 accessory surface for the inferior arch of the atlas, which is to 

 be found in the camel. The ex-occipitals are broad and low ; the 

 supra-occipital high and narrow, forming a prominent lamb- 

 doidal crest, and is continued well forward upon the roof of the 

 cranium. The shape of the occiput and especially of the crest 

 is rather more like the condition found in Camelus than in the 

 llama ; the median line is strongly prominent, caused partly by 

 the shape and position of the vermis of the cerebellum, partly 

 by a thickening of the bone in this region and development of a 

 diploetic structure. The foramen magnum is nearly circular in 

 outline, and differs from that of the recent species chiefly in the 

 character of the upper margin, which is more horizontally 

 directed, and in the median line is notched, with a slight projec- 

 tion on each side of it. The paroccipital processes are quite 

 long, broad, and very thin plates, ending in an obtuse point ; 

 they are flattened antero-posteriorly, and when seen from the 

 side present hardly more than their edges to view ; for most of 

 their length they are closely applied to the auditory bullae. 

 These paroccipital processes are placed more in advance of the 

 condyles than in the recent species, and are separated from 

 them by deep fossae. 



The basi-sphenoid is longer and narrower than the basi-occip- 

 ital, and, like it, is different in the two species, being decidedly 

 broader and more flattened in P. labiatam ; its anterior termi- 

 nation is just within the pterygoids. The presphenoid is so 

 much obscured by the long, sloping vomer that very little of it 

 is visible. The alisphenoids have narrow descending pterygoid 

 plates, which, compared with those of the llama and more es- 

 pecially the camel, are extremely short, and terminate in dis- 

 tinct hamular processes, which, however, are by no means so 

 prominent as in the modern species. The ascending portion of 



