286 A MANUAL OF DENTAL ANATOMY. 



where we should be justified in asserting that here the 

 homodont dentition has recently become heterodont : at 

 this point, for the first time, we have incisors, canines, 

 molars. 



As a matter of fact, a large number of extinct Ungulata 

 had the full typical number of mammalian teeth, viz., forty- 

 four, and in some the individual teeth, incisors, canines, 

 premolars, and molars, passed into one another by insensible 

 gradations, and contiguous teeth were but little differen- 

 tiated from one another. Professor Flower has described 

 afid figured such an extinct Ungulate under the name of 

 Homalodontotherium (Philos. Trans., 1874). It is exceedingly 

 interesting to find that back in geological time the dentitions 

 were more generalised, both carnivorous and herbivorous 

 mammals of the Eocene period usually possessing the full 

 typical number of teeth, and displaying less of special 

 modification ; but the few forms of life which have been 

 handed down in a fossil state do not as yet offer us by any 

 means an unbroken chain of forms differing from one another 

 by progressive modification, except in a few cases : thus the 

 ancestry of the horse is now comparatively completely known 

 to us. Bearing in mind that the several kinds of teeth have 

 probably a common origin, the homological differentiation 

 in the incisors, premolars, and molars may be advantageously 

 admitted, and made use of as a basis for comparing and 

 classifying the teeth of different animals. It is usually said 

 that when incisors are missing from the full typical number, 

 they are lost from the outer end of the series : that 

 is to say, if there is but one incisor it is I 1 ; if two, I x 

 and I . 



There are many exceptions to this : e.g., the first incisor 

 is the first to disappear in the otter, walrus, and some few 

 others. 



When premolars are missing, it is said that they are lost 

 from the front of the series. This is generally true, but the 



