21)8 



Mr. D. M. S. Watson on 



Structure of the Skull. 



Tlie basioccipital and basisplicnoid arc so fused tliat tlie 

 suture between them cannot l)c seen (fig. 3). 



The basioccipital part of the combined bone is probably 

 very sbort ; it is thin from above downwards, and its hiteral 

 borders are indistinguishably fused with the exoecipitals. 

 These latter bones are fused with the pcriotics, which form 

 one mass of bone in which no sutures are visible. 



The foramen magnum is surrounded by bone in which no 

 sutures are to be seen. The sutures separating this ring 

 from the surroundiug l)oues arc, liowever, quite clear 

 (fig. 4). 



Par 



F;g. 2. — Diademodon broiimi, R. 3587 and R. 3588. Side view. Refer- 

 ences as in tig'. 1, with V^i -m, foranieu for exit of sec(>nd and third 

 branches of the V. nerve ; 11, HI, IV, V', VI, notch for the exit of 



these nerves. 



The parocci[)ital processes, presumably belonging to the 

 opisthotics as in Spheiiodon, form tiie lower part of tlie back 

 of the skull; their ventral border is nearly straight, but (lij)S 

 down somewhat at the outer ends. '^IMieir posterior surface 

 is flat, and they are bounded above by a notch which forms 

 the lower border of the very small post-tcmpcral fossa ; on 

 the inner side of the notch they are united by jagged suture 

 Avith the parictals. The paroccipital processes arc of trian- 

 gular section, their forward face being excavated by a groove 

 which forms the posterior wall of the tympanic cavity (tig. 3). 



i 



