THE SOUTHERN SEA LION 



285 



YEAR OF AGE 

 1st 2nd . 3rd . 4th . 5th Bthohover 



muscles now cover the whole of the sides and top of the cranium. The pterygoids and 

 palatines project much farther below the level of the bony palate and the squamosals 

 are fused to the supra-occipitals. The canines much exceed the outer incisors in length 

 (Plates VIII, IX, X, figs. 6). 



The ADULT skull, that from an animal in its sixth year at the youngest, has an average 

 length of 344-9 mm. (31 specimens); the proportion to body length is 14 per cent 

 (9 specimens), the zygomatic width 

 is 215-9 mm. and the hamulo-pre- 

 maxillar length is 241-2 mm., the two 

 last being 62-6 and 69-9 per cent of 

 the total length of the skull. 



In this stage the nasals are almost 

 quite flat, and in profile are in the 

 same straight line as the median 

 anterior prolongations of thefrontals, 

 which are now below the level of the 

 post-orbital processes. In profile the 

 line of the nasals and the anterior 

 parts of the frontals forms an angle 

 of 1 60 degrees or less with the dorsum 

 of the cranium proper, the point at 

 which the change of direction takes 

 place being situated approximately at 



Fig- 4- 



Male sea lions. Variations in proportions 

 during growth. 



the level of the posterior end of the ^^' ^'^u" ^^"8^^ as percentage of body length. 

 jugal. The profile of the upper "^^ "''^"'°;P'''^f'"^'''""g'^^^P^'''^^"^^g^°f^'^"'''^"^^^ 



° r 1 1 11 . r • , -^r: Zygomatic Width as percentage of skull length. 



surface or the skull as a whole is thus 



once more bent, instead of being straight as it was in the fifth year. This change of 



direction of the profile increases with age, being 150 degrees in the skull No. 335X of 



the National Collection ; it tends to be obscured, however, by the development of the 



sagittal crest. 



Other osteological characters of the adult are these : the frontal and parietal segments 

 are fused together, the alisphenoid is united to the palatine and the latter to the pterygoid, 

 which is in turn fused with the parietal, while the squamosal is fused to the surrounding 

 bones. It follows that no further growth of the cranium is possible. 



The supra-occipital crest develops greatly, sometimes to an enormous size, as in the 

 skull No. 335:v:, where it is 6 cm. in height. The sagittal crest also develops markedly 

 in its posterior part, but in the anterior the development is irregular. Tubercles of 

 irregular shape are nearly always developed at the junction of the parietal and frontal 

 on either side, and less often on the parietal just in front of the occipital crest. Every- 

 where the ridges and rugosities of the skull are strongly developed and the whole 

 structure impresses one with the idea of extreme strength. 



The premaxillae nearly always unite in front and later fuse with the maxillae, and 



