454 HENRY C. TRACY 



12). A conspicuous notch in the edge of these wings anteriorly 

 forms part of the margin of the foramen auditivum. The lateral 

 concavities in the sides of the bone form the median and ventral 

 walls of the occipital portion of the saccular cavities (fig. 3). 



The ventral side of the body of the bone forms a narrow, 

 longitudinal, slightly concave surface bounded on each side by a 

 broad ventrally projecting plate of bone (figs. 8 and 11), This 

 surface and the ventral plates form the roof and part of the sides, 

 respectively, of the occipital portion of the eye-muscle canal. 



The external surface of the bone is formed chiefly by the ex- 

 ternal surface of the ventral plates and to a slight extent by the 

 ventral surface of the lateral wings, joined by a few roughened 

 trabeculae of bone. 



S. The exoccipital bone 



For descriptive purposes, we may consider this bone as com- 

 posed chiefly of two flattened masses; one forms a part of the 

 vertical posterior face of the skull, at the sides of the foramen 

 magnum (fig. 4), the other forms the part of the ventral surface 

 of the skull next to the basioccipital (fig. 5). These are con- 

 tinuous posteriorly with a thin, curved plate of bone which is 

 applied to the lateral and dorsal sides of the condylar part of the 

 basioccipital. The vertical part of the bone has laterally on its 

 outer surface a rounded projecting ridge which is continuous 

 above with a similar ridge on the epiotic (fig. 1). This ridge is 

 the prominent angle where the posterior surface of the skull meets 

 the ventral and lateral surfaces, and it lodges the posterior semi- 

 circular canal of the membranous labyrinth. 



Arising obliquely from the cerebral surface of the posterior 

 portion of the exoccipital is a thin triangular plate {TPEXO, 

 fig. 4), which passes medially to meet the corresponding plate 

 from the other side just over the dorsal edge of the median, 

 vertical plate of the basioccipital where a synchondrosis unites 

 the three contiguous edges of bone (fig. 12). Anteriorly the 

 plate tapers to a point which ends in the middle line just back 

 of the prootic bone (fig. 3) ; posteriorly it extends to the condylar 



