lOO ALUS. [Vol. XVIII. 



at first on the internal surface of the exoccipitale, then be- 

 tween that bone and the supraoccipital, then on the temporal 

 interspace of cartilage, and finally on the cerebral surface of the 

 petrosal marks the further course of this canal, and also, beyond 

 it, that of the anterior semicircular canal. 



The Supraoccipital (SO) forms the median portion of the 

 hind end of the dorsal surface of the skull and the dorso-median 

 portion of its posterior surface; its two portions, one dorso- 

 superior and the other postero-inferior, lying at an angle of about 

 135° to each other. 



The dorsal portion of the bone is convex on it outer surface, 

 which forms part of the dorsal surface of the skull, and concave 

 on its inner surface, which forms part of the roof of the cerebral 

 cavity. The posterior half of this part of the bone is somewhat 

 semicircular in outline, and is considerably wider than the anterior 

 half, from which it is separated by sharp reentrant angles. The 

 lateral edge of each of the processes formed by these reentrant 

 angles bounds the mesial edge of the anterior portion of the corre- 

 sponding temporal interspace of cartilage. The anterior half of 

 the dorsal surface of the bone usually has three separate edges, 

 the anterior one of which is transverse in position and nearly 

 straight, and bounds the hind end of the median, post-epiphysial 

 interspace of cartilage. Postero-mesial to the hind end of the 

 temporal interspace of cartilage the supraoccipital adjoins, on 

 either side, the antero-mesial edge of the exoccipitale, from which 

 it is separated by a line of cartilage. Between the temporal and 

 post-epiphysial cartilaginous interspaces the lateral edge of the 

 supraoccipital, on each side, is free, being separated from the 

 mesial edge of the postorbital ossification by a large opening in the 

 roof of the primordial cranium. This opening, or fontanelle, 

 extends forward almost to the anterior end of the alisphenoid, its 

 anterior portion lying between the alisphenoid, laterally, and the 

 post-epiphysial interspace of cartilage, mesially. The anterior por- 

 tion of the fontanelle is covered externally by the hind end of the 

 frontal, the posterior portion being similarly covered by the 

 parietal ; the edges of those two bones both overlapping externally 

 the dorsal surface of the supraoccipital. 



