Chap. IV.] 



TEETH OF CETACEA. 



153 



special armature of the mouth by " whalebone ; " 



but indications are presented both by palaeontology 



and embryology which are sufficient to justify us in 



believing that this 



order was primi- 



tively provided 



with a heterodont 



dentition. While 



the dolphin has a 



number of teeth 



in both jaws ; the 



sperm-whale has 



well-developed 



teeth in the lower 



jaw only; the 



bottle-nosed whale 



has never more 



than four func- 



tional teeth, and in 



the narwal (Mono- 



don) it is the male 



only that has the 



ordinarily single 



tusk well de- 



veloped ; this may 



be as much as nine 



feet long. Among 



the whale-bone 



whales teeth are 



never more than 



rudimentary, but 



in the adult their 



place is taken by plates of " baleen," which are 



set nearly at right angles to the axis of the mouth, 



and have their free ends frayed out into a number 



of stiff hairs, which make a most efficient strainer; 



the whale, in taking into its enormous mouth a 





Lower Jaw. 



