ON THE TEETH OF MAMMALIA. 105 



incisors, of the upper jaw were wanting ; but, nevertheless, no zoological 

 principles, by which to be guided in the study of Mammalia, resulted 

 from these notices : the zoological value of the dental system remained 

 in obscurity. 



It is not designed here to trace the progressive increase of attention 

 which has been directed to the dental system, from Ray to the present 

 era : _much less to expatiate on the effects of that attention, so ma- 

 nifest in the scientific works of this epoch an epoch signalized by 

 the rapid progress of philosophy in every department. In some in- 

 stances, perhaps, too much weight has been attached to dental characters, 

 to the exclusion of those afforded by other organs. Hence, guided 

 by the anomalous condition of its teeth, Linnaeus and others have 

 referred the Aye-aye, of Madagascar, a lemurine animal, to the order 

 Rodentia, under the erroneous denomination of Sciurus Madagascariensis. 

 In like manner, Pallas, and various naturalists, regarded the Hyrax as a 

 rodent ; instances, however, of this mistaken and exclusive regard to 

 the teeth do not lessen the value of the characters they really present. 



Being instruments for seizing or collecting food, and also for reducing 

 it to a state in which it is fitted for transmission from the mouth to the 

 stomach the teeth, as must be evident, will vary materially, according to 

 the nature of the food to be subjected to their action ; and, consequently, 

 they furnish a clue to the instincts and manners of an animal ; for instinct 

 and organization have a mutual dependance upon each other. In 

 Mammalia, the teeth are placed in a series, along the edge of the upper 

 and lower jaw bones, so as to oppose one another, and are fixed in 

 cavities, or sockets, termed alveoli, from the bottom of which they 

 originally arose.* Their number varies, and, in a few species, they are 

 altogether wanting : in some, as the Sheep and Deer, they are exclusively 

 appropriated to the procuring and preparation of food ; in others, 

 however, as the Dog, Tiger, &c., they are also used as weapons of 

 offence. 



The teeth may be regarded as consisting of two parts ; viz., a 

 crown, which is the part external to the socket ; and a root, which is 

 the part remaining implanted in the socket. 



Examination proves, that the crown does not usually consist of one 

 homogeneous mass ; but of two, and sometimes three, different substances, 

 which have received distinct appellations. In ordinary cases, the crown 

 consists of a central substance, essentially resembling bone, termed 

 ivory, enclosed in an envelope of enamel. With respect to the third 

 substance, it is termed cortical, crusta petrosa, or ccementum ; and by 

 its intervention, like a cement, masses consisting of ivory, encased in 



* In other classes of Vertebrata, the situation of the teeth varies. Among fishes, for example, they 

 not only occur on the true maxillary bones, but on the palate, the vomer, the tongue, and on the gullet. 

 VOL. I. P 



