GENERAL STRUCTURE. J 249 



the single subcorneal tooth (c) situated between the premolars and 

 the incisors the canine ; the latter tooth being the first of those 

 borne by the maxilla. The same terms are applied to the corre- 

 sponding lower teeth, although it will be unnecessary here to indicate 

 how such serial correspondence is worked out. For the sake of 

 brevity such a dentition may be expressed by the numerical 



3 — 3 I — x 4 — 4 3 — 3 

 formula — I. ; C. ; Pm. ; M. = 44; but 



3—3 i— 1 4—4 3—3 



since the teeth of opposite sides of the jaws always correspond, such 



a formula may be further simplified into — 



I. 5; C- ; Pm. ^ ; M. ^ = 22X2=44. 

 3 1 4 3 



The individual teeth of each group are enumerated from before 

 backwards, and by such a formula as the following — viz., 

 I.i,I. 2, 1. 3, C, Pm. 1, Pm. 2, Pm. 3, Pm. 4, M. 1, M. 2, M. 3, 

 I. 1, I. 2, I. 3, C, Pm. 1, Pm. 2, Pm. 3, Pm. 4, M. 1, M. 2, M. 3, 

 — each individual tooth can be specially noted. Thus, for example, 

 pm. 1 will indicate the first upper premolar, and m. 3 the third, or 

 last, lower true molar. It will frequently, moreover, be convenient 

 to speak of the incisors and the canine collectively as the cutting-, 

 and of the premolars and true molars as the cheek-teeth. It is very 

 generally the case that when the true molars are reduced to less 

 than three it is the hinder tooth, or teeth, that disappear ; in the 

 premolars it is, however, frequently the anterior teeth that are want- 

 ing, although this is by no means invariably the case, and there are 

 instances known where the second and fourth disappear, while the 

 first and third remain. In the figure of the lower dentition of the 

 Chimpanzee given on page 1247, the two premolars are usually 

 reckoned as pm. 3 and pm. 4, but it is not certain that such is 

 really the case. Again, it has been suggested that the two incisors 

 found in that species, in common with other Primates, may be the 

 first and third of the typical Eutherian series of three, although other 

 authorities regard them as the second and third. It may also be ob- 

 served that in some groups, as the Carnivora, the specialised forms 

 tend to lose the molars and retain the full number of anterior teeth ; 

 while in others, like the Ungulates, the reverse condition obtains. 



The milk-dentition may be expressed by a similar formula with 

 the prefix of the letter M. to the symbols. The typical milk-series 



will thus be written as M.i. ^, M. c~, M.m. ? ; the three milk- 



3 1 3 



molars corresponding to the last three premolars of the permanent 

 series. In a few Ungulates, however, such as Tapirus, and some- 

 times Rhinoceros and Palceotherium, four milk-molars are developed. 



