242 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



pliins ; or truncate, as in sloths and some rodents. The form is 

 complicated in" two ways, viz., either by the folding of the sides, 

 as in Glyptodon and many rodents, as Arvicola, Castor, Lepus, 

 etc. ; or by the develoi)ment of tubercles on the crown, as in Mus, 

 Dicotyles, Homb, etc. Upon this basis are constructed the more 

 complex types of teeth exhibited by the various families of Ungu- 

 lata and some Eodetitia. as has been pointed out in the following 

 language : " The genus Uintatherium* has been shown to be a Pro- 

 boscidian, which combines some important features of the Peris- 

 sodadyla with those of its own order. . . . The number of such 

 characters was shown to be somewhat increased in Batlimodon, 

 which therefore stands still nearer to the common point of de- 

 parture of the two orders. This point is to be found in types still 

 nearer the clawed orders ( TJnguiculatd) in the number of their 

 digits (4, 5), and in which the transverse and longitudinal crests 

 of the molar teeth are broken up into tubercles more or less con- 

 nected, either type of dentition [i. e.. Proboscidian or Perissodac- 

 tyle] being derived according as such tubercles are expanded in 

 the transverse or longitudinal directions." f 



As is well known, the crowns of the superior molar teeth of 

 the higher Mammalia are supported on three roots, two of which 

 are external, and the third internal. The corresponding inferior 

 molars are supported on two roots, and are therefore, in so far, less 

 complex. But these two roots usually support four tubercles, two 

 to each, while the roots of the superior molars support directly 

 but one each. Hence, as Prof. Harrison Allen remarks, the 

 crown of the inferior molar is more complex than that of the su- 

 perior. \ 



In tracing the degrees of complication of the crowns of the 

 superior molars of Mammalia from the simple cone of the Ceta- 

 cean, the first step is seen in the Squalodon and teeth of similar 

 chai'acter. In these there are two roots, antero-posteriorly related. 

 In the Squalodont Portheodon gervaisii {Squalodon Auct.) the 

 third root is present. In Mammalia with but three tubercles in 

 the crown, e. g., Carnivora, the inner root has much the form of 

 one of the external ones. In many of those with four tubercles 

 of the crown, e. g., Quadrumana, the form of the internal root is 



* Eobasileus in the original. (Ed. 1886.) 



f " On the Primitive Types of Mammalia Educabilia," published by E. D. Cope, 

 May 6, 18*73. " Pi'Oceeding.g of the American Philosophical Society," 18Y3, p. 224. 

 X "Dental Cosmos," December, 1874. 



