298 A MANUAL OF DENTAL ANATOMY. 



for they designated the same groups of animals ; in the 

 same way heterodont was an equivalent for diphyodont. 



But although this is true of a large number of animals, it 

 is not true of all, and it becomes necessary to note some of 

 the exceptions. 



The nine-banded armadillo (Tatusia peba) is a true homo- 

 dont : its teeth are all very nearly alike, they are simple in 

 form, and they grow from persistent pulps. Yet it has 

 been shown by Rapp, Gervais, and Professor Flower, to 

 have a well developed set of milk teeth, retained until the 

 animal is of nearly full size. 



Thus it is a true diphyodont, at the same time that it is 

 a true homodont mammal. But no milk dentition has been 

 observed in the sloths, nor indeed at present has it been 

 seen in any other armadillo (except the doubtfully distinct 

 T. Klapperi) ; nor have milk teeth been found in any ceta- 

 cean, so that the rest of the homodont animals are, so far as 

 we know, really rnonophyodont. 



Nor is it absolutely true that monophyodonts are all 

 homodont : thus the rudimentary teeth of baleenoptera are 

 heterodont (see p. 311). 



Upon the whole, our information respecting the " milk " 

 or deciduous dentition is defective ; but much light has 

 been thrown upon the subject by the investigations of 

 Professor Flower (Journal of Anatomy and Physiology, 

 1869, and Transactions Odontological Society, 1871), of 

 whose papers I have made free use in this chapter. 



The perpetual replacement of teeth lost, or shed in regular 

 course, which characterises the dentition of fish and reptiles, 

 finds no parallel in the case of mammals, none of whom 

 develop more than two sets of teeth. 



Just as homodont mammals as a rule develop but one set 

 of teeth, so heterodont mammals as a rule develop two sets 

 of teeth, though exceptions to this rule may be found. 



The deciduous or milk set of teeth may be of any degree 



