THE TEETH OF SIRENIA. 347 



vertical succession is known to occur among them. There 

 are no incisors nor canines, but there are horny plates in 

 the front of the mouth like those of the Dugong. 



The extinct Rhytina, formerly abundant about Behring's 

 Straits, was altogether without teeth. 



It has been mentioned that the teeth of the Manatee are 

 tapiroid in external form ; they also possess peculiarities in 

 minute structure, which are unusual in mammalian teeth, but 

 which are common to them and to the Tapirs. In examining 

 some teeth, I found that the dentine, to all intents and pur- 

 poses, of the hard unvascular variety, was permeated by a 

 system of larger, or "vascular" canals, which were ar- 

 ranged with much regularity, and passed out from the pulp 

 cavity to the periphery of the dentine, where they commu- 

 nicated with one another. The dentinal tubes did not 

 radiate from these vascular canals ; they, so to speak, take 

 no notice of them, so that there is an ordinary unvascular 

 dentine with a system of capillary- convey ing channels as 

 well. It is interesting to find that the primd facie external 

 resemblance of the teeth of the Tapir is fully borne out by 

 minute histological structure, and it certainly suggests that 

 the resemblance is not accidental, but has some deeper 

 significance. 



The enamel of the Manatee is also somewhat remarkable 

 for the absolute straightness of its enamel prisms in many 

 parts of the tooth. 



The molar teeth of the Dugong consist of a central axis 

 of vaso-dentine, a much larger mass of ordinary unvascular 

 dentine, and a thick layer of cementum, but they do not 

 share the peculiarities of the Manatee's tooth. 



