902 



TEETH. 



(herbivorous Cetacea of Cuvier) differ in has been called in question.* I have, how- 

 many organic particulars from the Cetacea ever, discovered in specimens of the Malayan 



Dentition of the Dugong (Halicore indicus). 



proper*, and in none, perhaps, more strikingly 

 than in having both deciduous and permanent 

 teeth; this succession takes place, at least, 

 with regard to the upper incisors of the Du- 

 gon?, Jig. 575. 



These teeth project from the gum in the 

 male sex;, but neither upper nor lower in- 

 cisors are visible in the female.-j- The supe- 

 rior incisors are but two in number, in both 

 sexes ; in the male, they are moderately 

 long, subtriedral, slightly and equally curved, 

 of the same diameter from the base, and 

 deeply excavated to near the apex, which is 

 obliquely bevelled off to a sharp edge, like 

 the scalpriform teeth of the Rodentia. The 

 form and extent of the persistent pulp-cavity 

 of this tooth are shown in the figure of its 

 longitudinal section, in my " Odontography," 

 pi. 93. fig. 4. ; it becomes longer and more 

 pointed than in the permanent incisor of the 

 younger male (fig. 575, z). When fully de- 

 veloped, only the extremity of this tusk pro- 

 jects from the jaw, at least seven-eighths of 

 its extent being lodged in the socket, the 

 parietes of which are entire ; and the exterior 

 of the great premaxillary bones presents an 

 unbroken surface. In the female Dugong, 

 the growth of the permanent incisive tusks of 

 the upper jaw is arrested before they cut the 

 gum, and they remain through life concealed 

 in the premaxillaries ; the tusk is solid, is 

 about an inch shorter and less bent than 

 that of the male ; it is also irregularly cylin- 

 drical, longitudinally indented, and it gradually 

 diminishes to an obtuse rugged point ; the 

 base is suddenly expanded, bent obliquely 

 outwards, and presents a shallow excavation. 

 These were conjectured by Home to be the 

 " milk-tusks ; " they are, however, character- 

 istic of sex, not of age ; and the existence of 

 deciduous tusks at any period in the Dugong 



Dugong which I have dissected at the 

 Zoological Society, the true deciduous in- 

 cisors of the upper jaw (fig. 575, d i) co- 

 existing with the permanent ones (a). They 

 are much smaller than the permanent tusks 

 of the female, and are loosely inserted by one 

 extremity in conical sockets immediately an- 

 terior to those of the permanent tusks, ad- 

 hering by their opposite ends to the thick 

 tegumentary gum, which presented no out- 

 ward indication of their presence. 



When this gum was stripped off the bone, 

 the deciduous tusks came away with it ; and 

 this may account for their usual absence in 

 dried crania of immature Dugongs, in which, 

 nevertheless, their alveoli are generally suffi- 

 ciently conspicuous. True permanent in- 

 cisors are not developed in the lower jaw of 

 the Dugong ; those which are occasionally 

 found there are abortive remnants of the first 

 or deciduous series, which are not destined at 

 any time to rise above the gum (fig. 575, 

 d i 3.). 



The molar teeth of the Dugong resemble 

 those of the order Bruta in the total absence 

 of enamel, and of any constriction defining 

 the crown from the fangs. In the Malayan 

 species, only five molars (fig. 575, 1, 2, 3, 4-, 

 5.) are developed on each side of both jaws : 

 in the Australian Dugong six are developed ; 

 t. e. the Halicore indicus is characterised by 



the molar formula m. ^=20, whilst the 

 5 5 



f* f* 



Halicore amtralis has m. |-=24.f But in 



b D 



both species, the number is progressively re- 

 duced, by the shedding of the anterior and 



smaller molars, to m. | = |=8. The struc- 



< '& 



ture of these molar teeth is illustrated in 



* Proceedings of the Zoological Society of London, * Dr. Knox, Edinburgh Philosophical Transac- 

 1838, p. 40. tions, torn. xi. p. 389. 



f Proceedings of the Zool. Society, 1838, p. 41. f See my appendix to Juke's " Voyage of the Fly.' 



