PROCEEDINGS OF THE NATIONAL MUSEUM. ^ol. 57. 



in all other forms in which it is present, as far as I am aware, it is 

 furnished by the frontal and not the parietal. It may arise at the 

 junction of tliese two bones on the edge of the orbit, and the parietal 

 may contribute a share in its formation, but Rhyncliocyon is the only 

 form that I loiow in which the postorbital is furnished exclusively by 

 the parietal. In conjunction with this unique formation of the post- 

 orbital process there is another still more remarkable structure to be 

 noted just at this point. On the rim of the orbit, reaching from the 

 supraorbital notch of the frontal, and extending well down toward the 

 tip of the postorbital process, is a long slender bone separated by 

 distinct sutures on either side of the skuU from both the parietal and 

 frontal. This extra bone therefore forms the free projecting edge of 

 the orbital rim and overlaps the junction between the frontal and 

 parietal where they meet above the eye. In the specimen before 

 us these bones are symmetrically developed upon the two sides of 

 the skull. 



The occipital surface is well separated from the sides and top of the 

 skull by a well-defined though not unusually prominent occipital crest. 

 The foramen magnum is relatively large and the condyles well sepa- 

 rated. There is no trace of a paroccipital process. From the center 

 of the occipital crest a sharp spinous ridge descends toward the fora- 

 men magnum, corresponding to the nuchal spine and serving for the 

 attachment of the nuchal ligament. There is a considerable exposm-e 

 of the mastoid portion of the periotic on the postero-lateral aspect of 

 the occipital region, wliich extends as liigli up nearly as the top of the 

 squamo-parietal suture. 



Of the orbital cavity and the formation of the zygomatic or quad- 

 rato-jugal arch, there are some pomts of unusual interest and impor- 

 tance to note. The squamosal is of fairly good size, and extends well 

 up on the side of the brain case, overlapping the edges of the parietal 

 in the usual manner. The posterior root of the zygoma forms a wide 

 concave projection behind the eye cavity, and passes backwards to 

 become continuous with the descending branch of the occipital crest. 

 In front it is produced into a relatively stout process, which articu- 

 lates directly in advance of it with the bone which runs forward to 

 the maxillary. It does not override this latter bone in its articula- 

 tion, but abuts against it, being received into a more or less V-shaped 

 pocket. 



One of the most remarkable features in the composition of the jugal 

 arch is the presence of a separate and distinct bone, lying near the 

 anterior end of the zygomatic process of the squamosal, at its junction 

 with the element in advance of it already mentioned. It is of an 

 elongated splintlike form, extending forwards toward the maxiUary 

 and resting upon the two elements below it, presently to be described. 

 It terminates behind in a more or less trihedral swelling or process. 



