328 Mr. I). M. S. Watson on 



and species of Gomphognathus , Diastemodon dhnorphodon, 

 on the anterior part of the right ramus of the mandible. 

 This subgenus was defined as having a very long diastema in 

 the lower jaw, the hinder part of which bears the roots and 

 worn-down crowns of " premolars." 



The systematic study of the group is hindered by the 

 very great difficulty of distinguishing the signs of wear on 

 the teeth and the apparent changes in the character of the 

 teeth with age. 



No characters exist for separating Diademodon and 

 Gomphognathus, except the fact that the teeth in the 

 type of Diademodon are little, if at all, worn, whereas the 

 teeth of the type of Gomphognathus are worn flat. This 

 difference is merely due to age. 



The difference between Gomphognathus and Diastemodon 

 pointed out by Seeley is a genuine one, and when our 

 knowledge becomes more adequate the latter will probably 

 have to stand. At present, however, as the lower jaw 

 11. 3588 is intermediate between Gomphognathus kanne- 

 meyeri and Diastemodon dimorphodon in this character, I 

 think it most convenient to include the whole series of 

 species in the one genus Diademodon. 



Diademodon tetragonus, the type of tlie genus, and 

 D. brachytiara are so badly known as to be at present 

 unrecognizable. 



The most primitive of the remaining types of which the 

 maxillary dentition is known is D. browni, to which species 

 I refer the skull that has been described at length above. 



The reasons for this reference are : — 



1. Agreement in size. 



2. Agreement in the structure of the upper and lateral 

 surfaces so far as they can be made out in the type. 



3. The fact that the posterior borders of the secondary 

 plates of the palatines meet in an angle and not in a straight 

 line. 



4. Agreement in the relative sizes of the dental alveoli, 

 in the structure of the fourth molar from the back when 

 allowance is made for wear, and of the extreme tips of the 

 last molar. 



The dentition has been described above, and its important 

 systematic features are : — 



1st. The very small size of the inner lobe of the penulti- 

 mate molar. 



2nd. The fact that the third molar from the back is only 

 5 of the width of the one in front. 



