284 ANATOMY OF VERTEBRATES. 



Dugong the whole of the smaller extremity of the tusk is sur- 

 rounded by a thin coat of true enamel, which is covered by a 

 thinner stratum of cement. In the male's tusk the enamel, 

 though it may originally have capped the extremity, as in the 

 female's, yet, in the body of the tusk, it is laid only upon the 

 anterior convex, and on the lateral surfaces, but not upon the 

 posterior concave side of the tusk, which is thickly coated with 

 cement. This side, accordingly, is worn away obliquely when the 

 tusk comes into use, whilst the enamel maintains a sharp chisel- 

 like edge upon the anterior part of the protruded end of the tusk. 



The presence of abortive teeth concealed in the sockets of the 

 deflected part of the lower jaw of the Dugong, fig. 160, , z, d 

 (vol. ii.), offers an analogy with the rudimental dentition of the 

 upper jaw in the Cachalot, and of both jaws in the foetal Whales. 

 The arrested growth and concealment of the upper tusks in the 

 female Dugong, and the persistent pulp-cavity and projection of 

 the corresponding tusks in the male, are equally interesting repe- 

 titions of the phenomena manifested on a larger scale in the dental 

 system of the Narwhal. The simple implantation of the molar 

 teeth and their composition are paralleled in the teeth of the 

 Cachalot ; their difference of form, and the more complex shape 

 of the hindmost tooth, ib. b, are repetitions of characters which 

 were present in the dentition of the extinct Zeuglodun. The 

 coexistence of incisive tusks with molar teeth, and the successive 

 displacement of the smaller and more simple anterior ones by the 

 advance of larger and more complex grinders into the field of 

 attrition, already seem to sketch out peculiarities of dentition 

 which become established and attain their maximum in the Pro- 

 boscidian family (Elephants and Mastodons) of the Ungulates. 



The molars of the American Manatee are thirty-eight in 

 number, ten on each side of the upper jaw, and nine, at least, on 

 each side of the lower jaw ; but they are never simultaneously in 

 place and use. The first in both jaws is small and simple. 

 Beyond the second, the crowns in the upper jaw are square, and 

 support two transverse ridges with tri-tuberculate summits, 

 having also an anterior and posterior basal ridge ; each tooth is 

 implanted by three diverging roots, one on the inner and two on 

 the outer side : they increase in size very gradually, from the 

 foremost to the last. The crowns of the four or five anterior 

 molars of the lower jaw resemble those above, but the rest have 

 a large posterior tubercle ; they are all implanted by two fangs 

 which enlarge as they descend, and bifurcate at the extre- 

 mity ; the crowns are of moderate height, and project only a few 



