76 MR. D. M. S. WATSON ON THE 



ceecling more delicate, because owing to a Pitot effect more air 

 will be driven into the nasal cavity ; (2) a great increase in the 

 size of the septomaxillary foramen, possibly associated with a 

 further elaboration of Jacobson's organ ; (3) a great increase in 

 the size of the facial part of the septoin axilla— this may be due 

 to purely mechanical reasons. 



Another advance very clearly shown in Gorgonops is a great 

 deepening of the maxilla and a concurrent reduction of the 

 lachrymal, the prefrontal showing little reduction. 



The palate of Gorgonops is ad/anced in the great size of the 

 internal nares, in the width, depth, and backward extension of 

 the median groove, in the internarial bar being single, and in 

 the occurrence of a median vomer in the back of the palate. 



The very large size of the pterygoids and the forward position 

 of the roughened and possibly tooth-bearing areas on these bones 

 are primitive features. 



The basicranial region shows no structural detail. 



Scymnognathus ivhaitsi shows many resemblances to Gorgono])s 

 m its snout. 



It shows advances over Arctops in the thinner basioccipital, 

 smaller basisphenoidal tubera, and less massive paroccipital pro- 

 cesses. 



The brain-case is advanced over that of the Pelycosaur 

 Dimetrodon in the great forward extension of the pro-otic and 

 supraoccipital and the junction of the latter bone with the 

 epipterygoid ; a remarkable feature is the forward process of 

 the basisphenoid, which forms a floor to the brain-case in 

 advance of the pituitary fossa. 



The most striking advances over Arcto])s are the great reduc- 

 tion in width of the parietal region, the lengthening of the 

 parietals, and especially the enormous increase in spread of 

 the squamosals. The turning backward of the upper part of the 

 squamosal at the posterior margin of the temporal fossa makes 

 that opening even larger, and increases the length of certain 

 fibres of the temporal muscles. 



So far as known, similar differences separate Scymnognathus 

 from Gorgonops, the latter genus having much larger iron ta Is 

 than the former. 



By the great expansion of the width of the back of the skull, 

 the orbits of Scymnognatlms are made to look partly forward. 



Another small but important advance in Scymnognathus is 

 that certain fibres of the temporal muscle have secured an origin 

 from the dorsal surface of the parietal region. 



Scymnognathus is probably lej-s advanced than Arctops and 

 Gorgonops in its less vertical occiput. In the palate Scymno- 

 gnathus is probably less advanced than Gorgonops in the small 

 size of the median groove, and its restriction to the anterior 

 part of the palate and to a narrow space round the hinder end 

 of the posterior nares. 



The incompletely known Leptotrachelus shows an important 



