Mr. D. M. S. Watson on the Cynodontia. 523 



siderable changes in the structure of members of the group 

 took place during this period. 



Galesaurus and Cytwsuchus represent very nearly the same 

 morphological stage, being little more advanced than the 

 contemporary Gorgonopsid Arctognathus. 



Primitive features are: — 



1. The low wedge-shaped snout. 



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



3. The large facial exposure of the septomaxilla. 



4. The only slightly complicated molar teeth. 



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



at its attachment to the brain-case in Galesaurus. 



6. The mode of articulation of the quadrate complex in 



Galesaurus to be compared with Asthenauchenia. 



7. The relatively large quadrate and hinder part of the 



jaw. 

 Advanced features are: — 



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 cheek-teeth. 



Nythosaurus retains a primitive feature in its very large 

 reflected lamina from the angular. 



Thrinaxodon is in general more advanced than the pre- 

 ceding form, but retains as primitive features : — 



1. The facial exposure of the septomaxilla. 



2. The septomaxillary 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 the 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 the squamosal. 



5. The triangular occiput. 



