Mr. D. M. S. Wsitson on the Cynodonda. 523 



siderable changes in the structure of members o£ the group 

 took phxce during this period. 



GaJesauius and Cynoauchus represent very nearly the same 

 morj)hological stage, being little more advanced than 'the 

 contemporary Gorgonopsid Arctoguathus. 



Primitive features are:^ — 



1. The low wedge-shaped snout. 



2. The overhang of the anterior border of the nasal. 



3. The hirge facial exposure of the se[)tomaxina. 



4. I'he only slightly complicaied molar teeth. 



5. Tiie broad low occiput and the depth of the squamosal 



at its attachment to lihe brain-case in Galesaurus. 



6. The mode of articulation of the quadrate complex in 



Galesaurus to be compared with Asthenauclienia. 



7. The relatively large quadrate and hinder part of the 



jaw. 

 Advanced features aie : — 



1. The reduction of the parietal region to a sagittal crest. 



2. The presence of paired occipital condyles. 



3. The secondary palate. 



4. The increase in the number of chetk-teeth. 



Nythosaurus retains a primitive feature in its very large 

 reflected lamina from the angular. 



Thrinoxodoii is in general more advanced than the pre- 

 ceding form, but retains as j)rimitive features : — 



1. The facial exposure of the septomaxilla. 



2. The septomaxillaiy foramen opening outward. 



3. Large frontals and lachrimals. 



4. A long quadrate ramus of the pterygoid. 



5. A considerable forward extension of the pterygoid on 



the sides of the median groove of the palate. 



6. The small auditory groove. 

 It is advanced in : — 



1. The lack of an overhanging anterior border of the nasal. 



2. The powerful quadrate ramus of ihe epipterygoid. 



3. The position of the foramen jugulare on the lower 



surface. 



4. The insertion of the quadrate complex into two slits in 



the lower border of thu squamosal. 



5. The triangular occiput. 



