1901.] AFFINITIES OF UBENODON. 167 



between the prefrontal above and the jugal (ju.) below. It has a 

 large lachrymal canal opening within the orbit. 



The prefrontal (pr.f.) is a somewhat quadrate bone — three sides 

 beiug formed by the orbit, the frontal, and nasal respectively, and 

 the fourth side by the lachrymal aud maxilla. It forms a con- 

 siderable portion of the anterior and upper wall of the orbit. 



The frontals (//•.) lie between the orbits and are usually well 

 developed. In Udenodon greyi they are narrow ; in U. gracilis 

 almost exactly as broad as long. They usually form the borders 

 of the upper sides of the orbits. Each frontal articulates with 

 the nasal and prefrontal in front, and with the parietal and post- 

 frontal behind. 



The postfrontal (po.f.) f orms, as in Dicyaodon and many Theco- 

 donts, an outer limb which forms the postorbital arch, aud a posterior 

 which runs backwards by the side of the parietal. The postorbital 

 arch varies considerably in different species, but is usually rather 

 broad above, forming a sort of roof to part of the orbit. In the 

 middle it is generally narrow and round ; while inferiorly it 

 broadens out and articulates with the squamosal and the jugal. 

 From the upper end of the postorbital portion the posterior part 

 runs abruptly backwards, forming the greater part of the inner 

 wall of the temporal fossa, and to a great extent hiding the 

 parietal. Posteriorly it curves outwards a little aud meets the 

 squamosal (cf. text-fig. 10, p. 165). 



The parietals early unite to form a single bone, which though 

 of moderate size is very largely hidden by the postfrontals. Near 

 the centre of the bone is a fairly large, parietal foramen. 

 Posteriorly the parietal articulates in the middle with the inter- 

 parietal, and laterally with the squamosals (sq.). 



The jugal is rather a small bone (ju.) which forms the greater 

 part of the infraorbital arch. Anteriorly it forms a considerable 

 part of the wall of the orbit at its anterior and lower side, meeting 

 the lachrymal and being clasped by the maxilla. Internally, the 

 anterior part meets the palatine. The squamosal (sq.), which lies 

 on the outer side of the jugal in its middle and posterior regions, 

 to a large extent hides it from view. Where the jugal meets the 

 postfrontal it is fairly deep in some species, but in others only 

 slightly increased in depth. Posteriorly it flattens out and lies on 

 the inner side of the squamosal, forming with it the temporal 

 arch. 



The squamosal is by far the largest bone in the skull (cf *</., text- 

 fig. 10, p. 165). It comprises a large broad posterior portion which 

 descends Erom its union with the parietal and postfrontal, along 

 the outer border of the supra- and exoccipitals, to give articulation 

 to the quadrate, and an anterior branch, which springs from the 

 upper half of the posterior portion ami passes forwards to form 

 with the jugal the temporal arch. 



The quadrate (7.) consists of a broad llat part which lies against 

 the front of the descending portion of the squamosal, and a large 

 articular head. The articular face <»l the quadrate lias a deep 



