the Skull of Jy'md^^modon, 319 



median and lateral areas of the occiput^ and are further 

 remarkable i'or the possession of large backwardly directed 

 plates similar to those of Dicranozygoma leptuscelus, although 

 apparently much less developed. 



The occurrence oF a definite, long, narrow, interpterygoid 

 vacuity Avas correctly described by Seeley. 



The fragments of lower jaw preserved seem to show that 

 the surangular was long and the coronoid process very low. 



Some general considerations concerning the Structure of the 

 Cynodont Skull. 



The identifications of the bones which I have described 

 above depend on a comparison of the Cynodont skull with 

 those of other early and primitive reptiles. All the deter- 

 minations are of old standing, having mostly been made by 

 0\\en or Seeley. 



I follow Broom^s determination of the vomer as corre- 

 sponding to that of a mammal and being probably a modifi- 

 cation of the reptilian paraspheuoid associated with the 

 development of a secondary palate. It is worthy of note 

 that in the crocodilian genus Metriorhynchus there is a 

 median vomer (Leeds, 1908) which is probably also a para- 

 sphenoid. 



Tiie pterygoid is easily recognized, and comj)arison w^ith 

 other early reptiles with the primitive reptilian (Rhyncho- 

 cephaliau) palate sIioavs that its remarkably slender posterior 

 ramus and somewhat unusual articulation with the basi- 

 sphenoid are to be correlated with the great reduction of the 

 quadrate and of all the bones which lie below the base of the 

 brain. Theromus provides an intermediate condition. 



The transverse bone has its usual relations despite its 

 considerable reduction. 



The identification of the various foramina depends on a 

 comparison with Sphenodon and a turtle, the broad columella 

 cranii of the latter rendering it rather more similar to a 

 Cynodont in the structure of the anterior part of the brain- 

 case. Starting at the back we find that : — 



I. Nerve XII. issued by the notch on the rim of the 

 foramen magnum. 



II. Nerves XI., X., IX. issued through the large pit 

 which exactly corresponds iuits general relations with 

 the foramen jugnlare of Sphenodon. 



