22 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



ing the upper edge of the orbital rim, overlapping the junction of the 

 parietal and frontal, and passing down upon the outside of the depend- 

 ing process of the parietal, which goes to make up the principal part of 

 the postorbital process above. As aheady pointed out, Eiiyncliocyon is 

 in this respect wholly unique among the Mammalia, being the only 

 form which to my knowledge has retained in its entirety this primitive 

 reptilian featm-e, as dis})la3^ed so perfectly by the extinct reptilian 

 genus above mentioned. I shall refer to this subject on a subsequent 

 page. Of the exact homolog}" of this element in the two forms, I do 

 not think, therefore, that there can be the slightest question or the 

 faintest doubt. 



If the element above considered be the true representative of the 

 poSifrontal of the reptilian skull, then the element lying below it in 

 the jugal arch, form-ing its postorbital process, can be no other than 

 the 'postorbital bone, sincp its position is almost if not exactly the 

 same as in ProcolopJion. Its form, moreover, when compared with 

 the corresponding element in this extinct reptile, is seen to be strik- 

 ingly similar. Its relations at its lower end are almost exactly alike 

 in the two forms, lying upon the upper side of the arch, near the 

 junction of the squamosal and jugal. In Rhyncliocyon it terminates 

 above in a free extremity, while in Procoloimon it extends upward to 

 join the depending process of the parietal (postorbital process) and 

 the squamosal. It would require but a slight extension of the bone 

 upward in Rhynchocyon to meet this downward projecting process 

 of the parietal to produce almost the exact relations above as found 

 in ProcolopJion. It is of course to be taken into consideration that 

 the change from the reptilian to the mammalian condition has pro- 

 duced considerable alteration in relations of certain parts of the 

 squamosal. In Rlnjnchocyon the upper part of the postorhital is 

 reduced and it has lost all contact with the squamosal above, as 

 shown in ProcolopJion. That it was formerly larger in RJiyncliocyon 

 or its ancestors and may have joined either the postfrontal or the 

 parietal, or both, is not altogether unlikely, but whatever the former 

 relations of its upper end may have been does not affect the main 

 question of its homology with the postorbital element of the reptilian 

 skull. 



In a like manner the element composing the lower part of the arch, 

 and extending from the maxillary in front to the glenoid cavity 

 behind, I homologize with the quadrato-jugal element of reptilian 

 anatomy, for the reason that its general and usual relations among 

 the Keptilia are of this character. It thus has the squamosal above, 

 the quadrate below, behind, and to the inner side, and the jugal in 

 front and above. With the disappearance of the quadrate it has 

 been shifted forward somewhat and has developed a connection 

 with the maxillary, which it does not seem to have in the reptilian 



