52 A BOOK OF~ WHALES 



nature, being useless they disappear. The enormous 

 size of the supra-occipital bone reduces the size of 

 the frontals with which it articulates. The latter are 

 very narrow above where they form the forehead, and 

 expand below where they protect the small orbit from 

 above. 



The pre-maxillary bones are remarkable for two 

 peculiarities. In the first place they do not, except 

 in some of the extinct forms (Zeuglodonts), bear any 

 teeth ; but, in the second place, instead of having 

 degenerated in bulk in consequence, they are greatly 

 increased ; they stretch backwards and touch, or 

 indeed partly cover, the frontals. The small size of 

 the nasals, which are almost rudimentary in all exist- 

 ing whales, and especially so in the Oclontocetes, 

 permits this junction to be effected. Laterally these 

 pre-maxillary bones are ensheathed by the maxillae, 

 a feature very characteristic of the whales, that is to 

 say, of existing forms. The maxillae also cover over 

 the frontals, and in some Odontocetes are greatly 

 crested on their dorsal surface, a feature which is 

 carried to a maximum in Hyperoodon and in the 

 Gangetic Platanista. 



The bones related to the organ of hearing are 

 extremely strong and stony in the whale tribe ; they 

 are imperfectly attached, as a rule, to the sur- 

 rounding portions of the skull, and are thus readily 

 detachable ; they are often found in a fossil condition 

 quite separate. The tympanic bone (Fig. 12) has a 

 shell-like form, not unlike a cowrie ; it is not always 

 firmly attached to the periotic, which ensheathes the 

 actual organ of hearing. 



