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On. the Homologies and Origin of the Types of Molar Teeth of Mammalia Educabilia. 



By Edward D, Cope, A.M. 



I. The types of Mammalian Molars. 



It has been already stated* that the transition from simple to complex teeth is 

 accomplished by repetition of the type of the former in different directions. " In 

 the cetaceans this occurs in the Squalodonts ; the cylindric incisors are followed 

 by flattened ones, then by others grooved in the fang, and then by two-rooted, but 

 never by double-crowned teeth. This is the result of antero-posterior repetitive 

 acceleration of the simple cylindric dental type of the ordinary toothed cetacean. 

 Another mode of dental complication is by lateral repetition. Thus the heel of 

 the sectorial tooth of a carnivore is supported by a fang alongside of the usual 

 posterior support of a premolar, and is the result of a repetitive effort of growth- 

 force in a transverse direction. More complex teeth, as the tubercular molars, 

 merely exhibit an additional lateral repetition, and sometimes additional longi- 

 tudinal ones. As is well known, the four tubercles of the human molar commence 

 as similar separated knobs on the [primitive] dental papilla." 



Accordingly the simple tubercle may be regarded as the least specialized form 

 of tooth. It may be low and obtuse, as in the Chiromgs, or walrus ; more elevated 

 and conic, as in the dolphins ; 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 

 Gli/ptodon and many rodents, as Arvicola, Castor, Lepus, etc. ; or by the develop- 

 ment of tubercles on the crown, as in Miis, Dicotyles, Homo, etc. Upon this basis 

 are constructed the more complex types of teeth exhibited by the various families 

 of TJngulata and some Rodentia, as has been pointed out in the following language: 

 " The genus Eohasileus has been shown to be a Proboscidian which combines some 

 important features of the Perissodactyla with those of its own order. . . . The 

 number of such characters was shown to be somewhat increased in Bathmodon, 

 which therefore stands still nearer to the common point of departure of the two 

 orders. This point is to be found in types still nearer the clawed orders {Ungui- 



* Method of Creation, p. 10, Philadelphia, 1871. 



