ii6 Elementary Zoology. 



case of the dogfish also we have simply to do with an involuted 

 part of the skin, i.e. the mouth cavity, whose lining epidermis 

 and dermis has retained the functions of those two layers, 

 elsewhere. Just as the teeth are in most animals attached to 

 bones of membranous, not cartilaginous origin, so the base of 

 these scales are ossified. Each scale, therefore, of a shark is 

 literally a tooth attached to a small bone. 



The comparison of teeth with scales is supposed to hold good up to the 

 Mammalia. But it must be borne in mind that the mammalian tooth 

 consists, in addition to enamel and dentine, of a layer of bone, the 

 so-called cementum. This may be even preformed in cartilage. It has 

 nothing whatever to do with the bone of the jaw to wliich the teeth 

 ultimately become attached. This general statement, therefore, "The 

 bones around the mouth have been recognised as havfng their origin in 

 tooth-bearing plates derived from fused placoid plates," must be, possibly, 

 somewhat cautiously accepted. It looks, in the case of the Mammalia, 

 at any rate, as if the dentary and other membrane bones which bear the 

 teeth were not the precise equivalents of the fused bony bases of placoid 

 scales, since the homologues of the latter exist (?) in the cementum of 

 each tooth. 



Teeth are found in- the frog and in the mammal; but 

 there is no bird in which these structures occur living at 

 present. There were formerly toothed birds; and some of 

 their descendants of to-day have retained rudiments, shown 

 during the development of the jaws, which are regarded by 

 some, though not by all, anatomists, as rudimentary tooth 

 germs. 



In the frog the teeth are developed upon the maxillag, 

 premaxilte, and vomers; in the rabbit, upon the two former 

 bones and upon the dentary of the lower jaw. The teeth of 

 the frog are all of approximately the same shape and size; 

 they are, moreover, very numerous, and fresh teeth are formed 

 when the first ones get worn out. In the rabbit, on the 

 contrary, the teeth are fixed in number, and only a few ; they 

 are varied in form,^ the chisel-like incisors being easily 

 distinguishable from the flat grinding teeth, while most 

 mammals have, in addition, the sharper canines lying between 

 the incisors and premolars. Furthermore, there are only two 



' " Homodont " and " Heterodont " are the terms used to express the 

 condition of the teeth in the frog and rabbit respectively. 



