298 



Mr. D. M. S. Watson on 



Structure of the Skull. 



The basioccipital and basisplienoid are so fused tliat tlie 

 suture between them cannot be seen (fig. 3). 



The basioccipital part of the combined bone is probably 

 very short; it is thin from above downer ards, and its lateral 

 borders are indistinguishably fused with the exoccipitals. 

 These latter bones are fused with the periotics, 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 surrounding bones are^ however, quite clear 

 (tig. 4). 



Par 



Fig. 2. — Diudemodon hroicni, R. S^jS/ and R. 3588. Side view. Refer- 

 ences as in fi«'. 1, with Vn-m, foramen for exit of second and third 

 branches of the V. nerve ; II, III, IV, Yi, VI, notch for the exit of 

 these nerves. 



The paroccipital processes, presumably belonging to the 

 opisthotics as in Sphenodon, form the lower part of the back 

 of the skull ; their ventral border is nearly straight, but dips 

 down somewhat at the outer ends. Their posterior surface 

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

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

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

 Avith the parietals. The paroccipital processes are of trian- 

 gular section, their forward face being excavated by a groove 

 wiiich forms the posterior wall of the tympanic cavity (fig. 3). 



