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that of the foramen jugularis to open by a single aperture on the 

 back of the skull just to the outer side of the condyle. On the inside 

 of the brain case the bone has a straight suture with the supraocci- 

 pital-epiotic, the lateral side of the bone is expanded into a flat plate 

 the lower border of which is thick and forms the stout paroccipital 

 process, separated off from the rest of the occipital plate by a post- 

 temporal fossa largest in Endothiodon and smallest in Lystrosaurus. 

 The lower surface has a stout process, no doubt entirely opisthotic, 

 which forms half of the fenestra ovale. The upper edge of the lateral 

 part of the bone has a long straight suture with the supra-occipital, 

 and nearly the whole of the front of the bone is covered by the pro- 

 otic. 



FOv BOc V" ExOc+OpO. 



Fig. 1. Occipital and otic bones of Lystrosaurus sp. posterior aspect. X^l^.B.Oc. 

 = Basioccipital. Ex. Oc. + Op. 0. = Exoccipital -j- opistiiotic. F. Ov. =^ Fenestra 

 ovale. S. Oc. + Ep. O. = Supraoccipital -{- Epiotics. IX. X. XI. XII. = foramen 

 for exit of those nerves. 



Fig. 2. The same specimen as fig. 1 viewed from the front. X ^2- Reference 

 letters as before with. B. Sp. ■= Basisphenoid. Pr. 0. =: Prootic. 771=: foramen 

 for that nerve. 



The pro-otic when viewed from within articulates with the 

 supraoccipital-epiotic and the opisthotic in the usual triradiate 

 suture; it forms the front of the internal auditory meatus, immedia- 

 tely in front of which it is pierced by the foramen for the Vllth 

 nerve: the anterior border of the bone has a notch over which the 

 Vth nerve left the brain case, below which is a distinct processus 

 anterior-inferior, which in all the genera except Lystrosaurus is 

 supported by the basisphenoid. The lateral border of the bone is 

 expanded into a thin film which covers the front face of the opisthotic, 

 and forms part of the paroccipital process. The lower surface of 

 the bone is produced into a downwardly directed process which 



