32 2 REV. S. GEAHAM BIRKS 



of the cranium is longer than the fronto-ethmoidal. The thick 

 membrane bones or dermal plates with their punctate layer 

 of ganoin exhibit a bilateral symmetry except towards the end 

 of the rostrum. In the anterior region the bones about the 

 median line are fused together in the adult to form the com- 

 pound ethmoidal shield. 



A median suture runs from the centre of the posterior 

 margin of this shield to the posterior limit of the roof bones 

 of the skull. This suture first separates the frontals along the 

 middle line ; the inner sides of these bones are the longest 

 and the suture dividing them is irregular. The frontals 

 are separated posteriorly from the elongated parietals by 

 a narrow area filled in by somewhat irregular and variable 

 processes and small bones. Each frontal exhibits a canal 

 with a curved course upon its surface. There is no median 

 frontal foramen. 



The parietals are long and narrow bones whose posterior 

 margins are about twice as broad as their anterior borders. 

 Each parietal is incised by a T-shaped canal. 



Either parietal is flanked on its external edge by two bony 

 elements which tend to fuse in the adult. The anterior 

 or post-frontal element is a narrow strip of bone which is 

 either partially fused with the posterior or squamosal element 

 or completely separated from it by an irregular curved suture. 

 The squamosal element is furnished with a transverse canal. 



Posterior to the parietais and to the squamosal elements of 

 their flanking bones is a pair of broad nuchal plates or supra- 

 temporals, one on either side, meeting in a median suture and 

 interposed between the large principal operculars. 



The sub-orbitals are three in number on either side of the 

 head ; they form the lower border of the anteriorly placed 

 orbits, of which the compound ethmoidal shield and the 

 frontals are the other investing elements. 



The maxilla is of an elongate triangular shape, the alveolar 

 border being the longest and the posterior the shortest. The 



