lO 



THE MAMMALIA. 



in the lower. Accordingly two rows of figures are 

 adopted for the two jaws, and the numbers are 

 arranged in accordance with the terms explained 

 above. Suppose, then, the reader meet with the 



following formula: — — 



I .4- 3_ 



- ='20. This is 



= 44 : how would 

 3-0.3-4 



he have to understand it .■' The row above the line 



gives the teeth standing in one half of the upper 



jaw ; and the animal thus has in each half of that 



jaw four incisors, one canine, four premolars, and 



three molars, in all twelve teeth, or twenty-four in 



the whole jaw. In the lower jaw, on the other 



hand, there are only three incisors, no canine, three 



premolars, but four molars, accordingly ten in all 



in each half of the jaw, or twenty in the whole jaw. 



For this animal consequently the total number 



of teeth amounts to forty-four, and since all the 



sorts of teeth are here represented the dentition is 



called a complete one. If the premolars and 



molars cannot be distinguished they are included 



in a single figure, as thus: '- — 



" " 1.0.4 



the formula of a rodent, and intimates that the 

 animal has one incisor above and below in each 

 half of the jaw, and four molars which cannot be 

 more particularly discriminated, and that in both 

 jaws the canine is wanting. 



But the dentition may be altered not merely by 

 the substitution of a permanent for a milk set, but 

 also by another process, namely by the premature 

 disappearance of certain teeth in the course of 

 years. At bottom this phenomenon is only a con- 

 tinuation, or the carrying out to a further stage of 

 that already mentioned, the non-development of 

 teeth the rudiments of which appear in the gum. 

 This premature disappearance is a very frequent 

 occurrence, and is observed especially in the more 

 recent mammals when compared with their im- 

 mediate predecessors, which retained throughout 

 life certain teeth which their descendants lose after 

 a short period. 



Evolution. — These facts lead us into a higher 

 region, that of the gradual perfecting of the 

 dental systems during the evolution of the ances- 

 tors of the present mammals carried on through 

 past geological epochs. Now what is this pro- 

 cess of evolution, and by what facts is it recog- 

 nized .' 



The answer to this question is not by any means 

 easy to give. So far we know only a few large 

 general features of the picture which it will remain 

 for the future to complete. For the present we will 



only briefly summarize a few of the most important 

 facts. 



We arc not acquainted with any mammals 

 belonging to a very early period with simple 

 conical teeth altogether unspecialized. We know, 

 however, some mammals now living, and among 

 these are included the dolphins and the giant 

 armadillo, which possess such teeth in almost un- 

 limited number — it may be a hundred or more. 

 Now these teeth, like those of the lower vertebrates, 

 are all similar, and are set so far apart from each 

 other that, when the mouth is closed, the teeth of 

 the one jaw fit into the gaps left between those of 

 the other. 



As soon as the teeth begin to exhibit a more 

 specialized form, their number becomes limited. 

 Yet in the older mammals this number is still 

 considerable, and some of the marsupials now 

 living have retained this character, the possession 

 of a large number of teeth. We have only a few 

 lower jaws of marsupials belonging to Triassic and 

 Jurassic times, and these are equipped with an 

 unusually large number of teeth. Dromatherium 

 from the Trias has fourteen, Amphitherium from 

 the Stonesfield Slates (Oolitic series of the Jurassic 

 strata) sixteen, Phascolotherium, from the same 

 slates, eleven teeth in a single half of the lower 

 jaw. Now, if we assume that the same number 

 was present in the upper jaw, then we get totals of 

 56, 64, and 44 teeth. The fauna of the present 

 day includes the banded ant-eater (Myrmecobius) 

 with 54, the opossums with 50, the bandicoots 

 (Peramelida) with 48, the Tasmanian wolf (Thyla- 

 cinus) and the genus Phascogale with 46 teeth. 

 The Monodelphia of the Tertiary period have all 

 44 teeth, and among the Monodelphia of the 

 present day there is only a single genus, Otocyon, 

 belonging to the Carnivora, in which this number 

 is exceeded. In it there are 46 teeth. 



But all these teeth are specialized. Fully de- 

 veloped incisors, canines, premolars, and molars 

 can be distinguished even in the oldest mammal 

 of the Trias, the Dromatherium, and if the front 

 teeth stand more or less apart, the back ones begin 

 to press more and more closely on one another, 

 and show the characteristic double fangs. We can 

 trace these combined modifications in following 

 out the evolution of a particular series. As the 

 dentition becomes transformed from the general 

 omnivorous or insectivorous type to one of a more 

 special character we see the teeth reduced in 

 number, and at the same time more and more 



