THE TEETH OF CETACEA. 311 



more considerable dimensions. All trace of this second pair 

 of incisors is lost in adult skulls. 



The Cetaceans classed together as Ziphoids have no teeth 

 in the upper jaw, and in the lower jaw only two (in a 

 single species there are four) teeth which attain to any 

 considerable size, though other rudimentary teeth have been 

 formed in the dense gum. 



The structure of these teeth is very peculiar : a tooth of 

 a species of Ziphius in the Oxford University museum, 

 which was described by Professor Ray Lankester, consists in 

 great part of cementum and osteo-dentine, the true dentine 

 being merely a little fragment situated at the top, and not 

 forming more than a tenth of the whole bulk of the tooth. 



Ziphius Layardii has teeth nearly a foot long projecting 

 upwards from the lower jaw, which arch towards one 

 another, above the upper jaw, so that they must prevent 

 the mouth from being opened to any considerable extent. 



The whalebone whales are, in the adult condition, des- 

 titute of teeth, but prior to birth the margins of both upper 

 mid lower jaws are covered with a series of nearly globular 

 rudimentary teeth, which become calcined, but are speedily 

 shed, or, rather, absorbed. 



The foetal teeth of the Balsenoptera rostrata have been 

 carefully described by M. Julin (Archives de Biologic, 1880), 

 the Balcenopteridce having been previously supposed to be 

 without rudimentary teeth. The ramus contained 41 tooth 

 germs, each furnished with an enamel organ and dentine 

 bulb, with a slight capsule; these were lodged in a con- 

 tinuous groove in the bone above the vessels, thus re- 

 calling the condition of the parts in a human embryo at a 

 certain stage. A very small amount of calcification takes 

 place, a mere film of dentine being formed upon the dentine 

 bulb. But what is very remarkable is that the dentine 

 bulbs are simple near the front, bifid in the middle, and 

 trifid at the back of the mouth ; in other words, these 



