ZOOLOGY AND PALAEONTOLOGY 189 



regular in this series, from the earliest to the latest 

 form ; while the structure of the molars also pre- 

 sents a complication equally progressive. In the 

 angustidens species and its mutations, the molars 

 which precede the last are composed of three 

 transverse ranges of mounds, whence the term trilo- 

 phodont structure, which has sometimes been raised 

 to the rank of a sub-genus, under the name of Trilo- 

 phodon. In the longirostris species these molars 

 have four ranges of mounds, whence the sub-generic 

 name of Tetralophodon. But, in studying the 

 Mastodons of this branch, which we meet with in 

 the highest strata of the middle Miocene at Ville- 

 franche d'Astarac in the Gers, for example we 

 note that the heel of the last molar is doubled, 

 so that we may reckon, if we will, one mound the 

 more ; and, owing to this intermediate mutation, 

 the distinction between the genus trilophodon and 

 the genus Tetralophodon becomes quite illusory. 

 On the other hand, in the small forms of the lower 

 Miocene, the last range of mounds diminishes in 

 importance with the last molar, and may be con- 

 sidered in certain specimens as a simple heel to the 

 tooth ; a Mastodon in which the last molar thus 

 only carries three ranges of mounds approaches 

 visibly the normal type of structure of the genus 

 Palceomastodon of the middle Oligocene of the 

 Fay urn. Yet in the present state of our knowledge 

 this genus can still be easily separated from the 

 true Mastodons by its puny size, by its much 

 simpler last molar, and, above all, by the presence 

 of five molars in simultaneous function in each jaw. 

 But this facility of diagnosis of the two genera is 



