ON THE TEETH OF MAMMALIA. 109 



remark, however, cannot be omitted, namely, " that, through the endless 

 diversity, which the microscopic texture of the teeth of different animals 

 presents, the universal law of the tubular structure can be unequivocally 

 traced ; and, that the general tendency of the modifications observable, in 

 descending from Man to the lower classes of the vertebrate animals, is a 

 nearer approximation of the substance of the tooth to the vascular and 

 organized texture of bone." With reference to the application of the 

 tubular structure of the teeth, to the explanation of their pathology, a 

 new field, as Professor Owen observes, is opened ; from the cultivation 

 of which some valuable practical improvements in dental surgery may 

 result. In the use of the microscope, for the examination of thin 

 slices of fossil teeth, with a view to the determination of the natural 

 group, to which the animal itself belonged, when other characters fail, 

 or when a complete tooth is unattainable, we have a new and impor- 

 tant test. 



With respect to those groups of Mammalia, in which the teeth 

 present the usual structure of compact ivory, enamel, and cement, 

 they have been minutely described in several genera, by Professor 

 Retzius. 



The bearing, however, of the microscopic discoveries in dental 

 structure, upon the natural history of the Mammalia, is rather remote than 

 immediate : to the naturalist, the forms, the number, and the arrange- 

 ment of the teeth, are of primary importance ; and it is upon these points, 

 conjointly with others, that his groups of the Mammalia are founded : 

 nevertheless, he is glad to receive aid in his pursuit, or confirmation as 

 to the justness of his views, from the researches of the microscopic 

 anatomist, who, in his investigations of the intimate structure of the 

 organs in question, traces modifications exclusively peculiar to the 

 natural families, or genera, established on other data. 



An examination may now be made into the general characters, which 

 the roots, or the fangs, of the teeth present, as to their relative degree 

 of perfection. 



The roots of teeth are either real or fictitious. In the first case, 

 the root consists of either ivory, solely (the enamel ending at the com- 

 mencement of the root, commonly called its neck), as in Man, the 

 Carnivora, Ruminantia, &c. ; or of ivory and coementum, as in the 

 Cachalot. When the root is fictitious, it is a mere continuation of the 

 crown, whatever that may consist of, with the same arrangement of its 

 parts. Teeth, with fictitious roots, continue to grow, at least to a late 

 period of life. The tusks of the Elephant, the incisor teeth of all rodents, 

 and the molars of many, as of the Hare, Cavy, and Arvicolae, are ex- 

 amples ; but not the molars of the Squirrels and true Muridae, or Rat 

 tribe. The formation of the teeth takes place in what anatomists have 



