Development of the Mammalian Phylum. 373 



have originally arisen through fusion. The further hypo- 

 theses of the American paleontologists, in connexion with the 

 tritubercular type of tooth, are not affected by this. 



A radical distinction would consequently have to be drawn 

 between the molars of the reptiles and those of the mammals. 

 The teeth of the theromorphous reptiles, whose molars were 

 already described by Owen in most cases as simple conical 

 teeth, are only homologous to a simple reptilian tooth, or else, 

 as in the case of the Theriodontla, a fusion takes place. This 

 fusion, however, always affects the individual tooth alone, 

 and the rudiment of its corresponding successional tooth, 

 which is contained in the dental fold. (My view is clearly 

 illustrated by the figure of tlie skull of Empedocles molan'sj 

 Cope, given by Zittel in his ' Handbuch der Paliiontologie,' 

 Bd. iii. p. 581.) The molars of the Mammalia, on the other 

 hand, represent much more complicated structures ; they 

 have arisen through the fusion of a larger or smaller number 

 of conical reptilian teeth which lie one behind the other, and 

 in addition to these there is usually added the corresponding 

 series of teeth of the second and it may be of the third den- 

 tition. In this process the shortening of the jaws must have 

 had an important mechanical effect. 



I would further support my hypothesis by the following 

 consideration, which also embraces the other classes of Verte- 

 brata. In the first place I lay doxon this principle for the 

 development of the teeth within the entire vertebrate series j 

 that the development of the dentition is primarily traceable to 

 the fusion of individual teeth. 



The simple dentine tooth of the fishes is to be regarded as 

 the primary element. Just as, according to O. Hertwig, the 

 covering bones of the oral cavity have arisen through the 

 growing together of the basal plates of these elementary 

 structures, so also through fusion of the teeth themselves 

 more complicated forms of teeth have been produced. 



This process can be traced by means of comparative 

 anatomy in the Selachians. Thus, for instance, Gladodus^ 

 one of the oldest forms of sharks, exhibits the following 

 arrangement of teeth : on an elongated base a number of 

 conical tips arise, of which the middle and the two outer ones 

 are the longest {vide Zittel, Bd. iii. p. 67). The origin of 

 this dental structure would be quite unintelligible if we 

 would assume it to have arisen through gradual differentia- 

 tion of a single tooth-tip ; it apjjcars, on the other hand, quite 

 natural to suppose this formation to consist of a series of 

 individual teeth fused together. The other forms of teeth 

 then arose throuo;h the more and more intimate fusion of the 



