20 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.57. 



younger stages, then I can not understand why it should be in the 

 present specimen partially, and I may say almost completely, sepa- 

 rated from it by suture which shows alike on the two sides. By its up- 

 per posterior extremity it articulates with the squamosal and exoccipi- 

 tal, there being no mastoid portion of the periotic exposed in this re- 

 gion of the skull that I can find. In front it is produced into a more or 

 less pointed free extremity directed forwards and inwards toward 

 the pterygoid. It is attached above to the periotic. 



Scarcely less interesting than the foregoing is the presence of a 

 relatively large foramen or vacuity passmg from behind forwards 

 just above the glenoid articulation, being bounded above and on the 

 outside by the squamosal, on the inner side by the exoccipital, and 

 below by the exoccipital and squamosal. The parietal above does 

 not enter into the formation of this vacuity, but reaches down 

 almost to its upper boundary. In the curious rodent LopMomys this 

 vacuity is represented in part. Instead of running forwards entirely 

 above the articulation, as it does in Ornithorliynclius, it enters just 

 back of the joint and has the periotic for its internal boundary. A 

 large venous foramen in the Marsupials in this situation may represent 

 the remnant of the same structure in these forms. 



Homologies of these suvernmnerary hones. — From a careful con- 

 sideration of the foregoing facts, what conclusions or deductions can 

 be drawn as to the hom-ologies of these elements, and what light do 

 they thi'ow upon the broader problems of the descent of the Mammalia 

 from the Batrachia or Reptilia? With the rather incomplete 

 embryological evidence which we have I think we are justified in 

 assuming that the occurrence of such characters in RhyncJiocyoji as 

 above described, even though they may be largely obliterated in the 

 adult skull, and are only occasionally to be met with in exceptional 

 specimens, is none the less very strong presumptive proof that these 

 vestigial structures represent separate and distinct elements in certain 

 mammalian skuUs, at least, which were once to be found in practically 

 aU stages and ages, just as in the Reptilia. As to the frequency of 

 the occurrence of these remarkable features of the skull in Rhynchocyon 

 1 have little or no additional evidence to offer, further than that 

 furnished by the figures of the skull by Peters ^ of RTiyncliocyon cirnei, 

 in which some of the sutures, at least, delimiting these elements are 

 represented. There can be little doubt that if his specimens were 

 carefully studied with the object in view of determining this point, 

 they would ofl^er conclusive confirmatory evidence of the facts above 

 set forth. At all events, in the light of such embryological testimony 

 as we have, respecting the presence of certain of these elements in the 

 Mammalia, we may, in my judgment, safely conclude that when 



I Naturwiss. Reise nach Mossambique, 1852, pi. 13. 



