CLASSIFICATION OF THE THEIMOUONTIA 43 



interesting features not previously recorded, I give here a more 

 detailed description. The premaxilla has a narrow dentigerous 

 surface, with five sockets for the roots of the incisors ; aboA'e the 

 border the bone thickens, forming a deep wall, from whose ad- 

 median half the palatine process arises ; this is at first a fiat 

 expansion, but soon becomes a rounded, backwardly directed 

 tubercle, separated from its fellow and resting in a groove on the 

 lower surface of the " prevomer." The internarial bar is a narrow 

 rod which anteriorly is comparatively wide. Anteriorly its 

 palatal surface bears a median ridge which separates two channels 

 bounded by other lower ridges, which form the lateral borders of 

 the bone. Further back the lower surface of the bar becomes 

 flat, and the whole of the posterior part is only represented by a 

 broken surface, which probably originally supported a deep 

 median ridge. 



The internal nares are very large openings bounded by the 

 premaxilla in front, where they are very wide, and contracted 

 posteriorly by the thickening of the maxilla? necessitated by 

 the large sockets for the canines. The maxilla? form their 

 outer borders for some distance and are then excluded by the 

 palatines. Finally, the posterior border is formed by the semi- 

 circular margin of a bone whose nature has to be discussed. 

 Between the internal nares and the pterygo-parasphenoidal bar 

 the palate forms a large area of complicated shape. The height 

 above the lower margin of the premaxilla at which the palatal 

 processes start, and the deep step in the lower border of the 

 maxilla just in front of the canine make the ventral surface of 

 the internarial bar lie much dorsal to the lower edges of the 

 maxilla? in the cheek-region. Thus at the back of the nares the 

 palate is very much vaulted. Behind the canine the palatal 

 exposure of the maxilla, which bears no trace of cheek-teeth, is 

 very broad and its admesial surface forms a deep vertical plate. 

 This surface when followed caudally passes into a, similar face 

 carried by the palatine, which stands almost vertically, tightly 

 attached to the maxilla by an obvious and deeply interdigitated 

 suture, and with its lower edge forming with that bone a broad flat 

 face in the area where cheek-teeth would naturally be expected. 

 These teeth must have been functionally replaced by a hard gum, 

 possibly cornified so as to form a crushing plate. 



The wide groove formed by the palate at the posterior end of 

 the internal n?res is rapidly divided into three, each groove of the 

 lateral pair is deep and narrow and cylindrical ; it shallows 

 rapidly when traced backwards, finally becoming flat when it 

 reaches the ectopterygoid. The bottom of the lateral groove 

 has a suture running the whole of its length, which is completely 

 exposed on the right side, but concealed by matrix except for its 

 anterior end on the left side of the type-skull. This suture, 

 which seems to be truly a suture and not a crack, unites the pala- 

 tine with the pterygoid, which bone hence forms the posterior 

 margin of the posterior nares. 



