No. 434.] THE AMER1C. 



7 RIA. 



95 



trodon by the loss of cartilage and the approximation of the 

 posterior portions of the pterygoids and the basioccipital and 

 basisphenoid bones. 



6. The development of the bipartite occipital condyle. 



Among the American forms there seems to be none which 

 approach the condition of a single zygomatic arch ; among the 

 African forms none has been discovered which shows the two 

 temporal arches at all perfectly. 1 The nearest approach to the 

 Rhyncocephalian condition is in Procolophon and Cynognathus, 

 as the structure of these two clearly indicate the previous pos- 

 session of two arches we may assume the existence of a Diopeus- 

 like ancestor as indicated above. 



I select as examples to illustrate the above conclusions Cynog- 

 nathus, Galesaurus and Gomphognathus because they are the 

 best preserved and described forms and because they arc typical 

 of the three fairly distinct groups, the Cynodontia, Lycosauria 

 and Gomphodontia. All of these have the well developed occip- 

 ital crest and cranial wall, but show the advancing stages of 



the teeth. 



eath ■ 



reaches far back and comes in contact with tl 

 of the united prosquamosal and squamosal (p 

 The quadrat o-jugal is probably crowded ben 



of the bones 1 of the two arches is accompan 

 tion of the two arches leaving only a sm 

 vacuity between the two and by the suppres 

 The psq + sq is described by Seeley (Seek 

 bent upon itself nearly at right angles, th< 

 extending outward and forming the posteri 

 The inner half of this plate, squamosal portic 

 to the cranial wall, and the outer, prosqua 



