4 ROYAL SOCIETY OF CANADA 



tion, and transversely convex on the upper surface, a rounded ridge 

 being thus formed that extends backward to near the posterior border. 

 From here an abrupt lateral extension of the bone to either side as a 

 robust transverse bar, concave in outline behind when seen from above, 

 constitutes the hinder margin from which are thrown off. at some dis- 

 tance on either side of the median line, the robust hook-shaped inwardly 

 directed processes that form so prominent a feature in the general 

 appearance of the crest. The posterior l)ar continues outward and for- 

 ward on either side as relatively thin, narrow extensions, which, finally 

 curving in to join the axial part anteriorly, completely enclose the large 

 fontanelles. These alar extensions slope downward at a considerable 

 angle, about 25°, on each side, and in an antero-posterior direction 



partake, though in a less degree, of the longitudinal concavity of the 

 median ridge. Along the median line the bone gradually thickens 

 backward until a maximum thickness of six centimetres is attained at 

 the centre of the posterior border. In the following figure, which repre- 

 sents the crest, as seen from above, one-eighth the natural size, the 

 thickness of the bone in different parts is indicated in centimetres. 

 From this it will be seen that the crest is decidedly robust behind, and 

 that the bone gradually thins toward the front. The posterior bar near 

 the median line presents a backwardly directed vertical face, wliich 

 becomes rounded and less robust in the neighbourhood of the 

 hooked processes; it is not, however, quite bilaterally symmetrical, 

 its transverse section near the left hooked process being nearly 



