Some New rrimitive Thcriodonts. 149 



other hand, there is no trace of a secondary palate, the internal 

 nares being situated in the anterior palatal region, and the general 

 arrangement of the bones being essentially similar to that found in 

 Sphenodon. 



The prevomers (" vomers " of most avithors) are a pair of elongated 

 bones which in front separate the internal nares, and posteriorly 

 form part of the hard palate. The anterior part of each articulates 

 with the short palatine process of the premaxillary. Towards the 

 posterior part of the internal nares each prevomer becomes flattened 

 into a thin vertical plate, but on reaching the posterior border of the 

 nasal passage each bone becomes flattened out transversely and 

 forms a large part of the hard palate. This posterior part of the 

 prevomer articulates laterally with the palatine and posteriorly with 

 the pterygoid bones. 



The palatines lie between the posterior part of the prevomers and 

 the maxillaries, and form about two-thirds of the hard palate. If a 

 transverse section were made a short distance behind the posterior 

 border of the internal nares the hard palate would be seen to have 

 the foi'm of an arch, forming almost a complete semicircle. The 

 lower margins of the maxillary bones are comparatively thin and are 

 supported by the palatine bones being closely articulated alongside. 

 From this articulation the palatines pass upwards and inwards to 

 meet the prevomers and backwards to meet the pterygoids and the 

 transpalatines. 



Each pterygoid may be described as consisting of three parts : an 

 anterior portion lying between the palatines and the prevomers ; a 

 transverse portion which meets the transpalatine ; and a posterior 

 portion, not present in the specimen, which was doubtless continued 

 to meet the quadrate. The anterior portion is in front about as 

 broad as the prevomer, but on passing backwards it gradually 

 becomes broader till the transverse portion is formed. A large oval 

 vacuity is present on each side bounded by the pterygoids and 

 palatines internally and by the transpalatine externally. On the 

 plane of the transverse processes of the pterygoids an oval vacuity 

 is present in the middle line between the two pterygoids. From 

 near the sides of this median vacuity there pass forwards and out- 

 wards on each side a row of very small teeth. 



The transpalatine or ectopterygoid is a comparatively slender bone 

 which articulates in front with the palatine, the jugal, and apparently 

 with the maxillary. On passing backwards it meets and supports the 

 transverse process of the pterygoid. 



The dentition of Scylacosaurus is particularly interesting. Each 



