HORNY TEETH 369 



the mouth grows gradually under the calcified teeth, a 

 method of growth which has possibly something to do with 

 the shedding of the latter '. 



The calcified teeth, which are all molars, consist of dentine 

 and enamel of somewhat imperfect structure, the dentine 

 being characterized by the immense number of interglobular 

 spaces present, which would appear to be an indication of 

 imperfect calcification, which is still more apparent in the 

 short roots ; these, as C. S. Tomes says, ' are of a softer, 

 coarser material than the crown, which itself is not of a high 

 type of dentine structure '. 



In the Tadpole, prior to the commencement of the forma- 

 tion of true calcified teeth, there are horny plates upon the 

 jaws, and on the inner margins of the lips are numerous 

 horny projections, each one of which is, according to 

 C. S. Tomes, the product of a single epithelial cell ; these 

 little conical teeth, as well as the larger plates on the jaws, 

 are continually being shed and renewed from beneath. 



In the Chelonia (Turtles and Tortoises) there are no 

 calcified teeth, but a horny casing covers both upper and 

 lower jaws, which is broad and crushing in the vegetable 

 feeders and elevated into a sharp ridge in the carnivorous 

 species, as in Cheloneimbricata (the Hawk's-bill turtle), where 

 this horny covering forms a hooked beak with a sharp 

 edge. 



Horny plates are also present in the jaws of the Sirenia 

 (Manatee and Dugong), which are considered to be allied 

 to the Ungulates, although formerly classed as Cetaceans. 

 These horny plates in the lower jaw of the Dugong cover 

 the abortive rudimentary calcified teeth which never become 

 functional. 



REFERENCES 



1. Ayres. Biological Lectures at Wood's Hall, 1894. 



2. Beard, J. (a) ' The Nature of the Teeth of Marsipobranch Fishes.' 



Morph. Jahrb., 1889, Bd. iii, pp. 727-53. 



(b) ' The Teeth of Myxinoid Fishes.' Anat. Anzeig., 1888, Bd. iii, 

 pp. 169-721. 



3. Bridge, T. W. Cambridge Natural History, vol. vii, p. 248. 



4. Howes. Nature, Nov. 1894. 



MUMMERY 



