MONOTREMES. 309 



CHAPTER II. 



HORNY TEETH. 



MONOTREMES. 



131. In entering upon the special consideration of the teeth 

 in the Mammalian class, we meet, at its lowest step, as in Reptiles 

 and Fishes, with horny substance in the place of teeth, properly 

 so called. In the Myxines these dental substitutes were of a 

 conical form, like canine teeth ; in the Siren the horny matter 

 sheathed the fore-part of the jaw-bones with a trenchant or incisive 

 plate ; in the Ornithorhynchus it assumes the form of both incisive 

 and molar teeth. These bodies, in the paradoxical Mammal, have 

 been admitted by M. F. Cuvier amongst the "Dents de Mara- 

 miferes," with good figures and descriptions of their outward form. 

 Heusinger has accurately described their intimate texture, and an 

 affirmative reply has been given, by the common consent of all 

 Anatomists and Naturalists who have treated of the Ornithorhynchus, 

 to the remarkable anticipatory remark with which Hunter concludes 

 one of his philosophic generalizations on the dental organs : — 

 " We may call," he writes in 1792, "everything teeth fitted for the 

 purposes before mentioned, (viz., holding or retaining, dividing, 

 cracking, and grinding the food), which is composed of an animal 

 substance and calcareous earth, called bone : how far horny sub- 

 stance may be so shaped as to deserve the name of teeth, I do 

 not yet know." Within ten years after the decease of Hunter, 

 Mr. Home was enabled to describe the most conspicuous of the 

 horny teeth of the Ornithorhynchus as follows : — " The teeth, if 

 they can be so called, are all grinders ; they are four in number, 

 situated in the posterior part of the mouth, one on each side of 

 the upper and under jaw, and have broad flattened crowns. In 

 the smaller specimens before examined, each of these large teeth 

 appeared to be made up of two smaller ones, distinct from each 

 other. The animal, therefore, most probably sheds its teeth as 

 it increases in size. They differ from common teeth very materially^ 



