1884.] O [Cope. 



though in the Indian species, at all events, it appears to be always con- 

 stant in the milk-molars. We have seen that there is a tendency in the 

 true molars of some of the Trilophodons (M. falconeri) to develop the 

 talon into a fourth ridge, and in the Tetralophodons (M. latidens and M. 

 sivalensis), a similar talon is developed into a fifth ridge, in the intermedi- 

 ate true molars." M. liumboldtii Cuv. (if. andium Falc.*) shows a small 

 fourth crest on the second true molar, according to Falconer. f 



The lower incisor teeth, on which Godman relied for the definition of 

 his genus Tetracaulodon, were shown by Harlan, not to be constantly 

 present in the Mastodon americanus. In fact, no adult specimen has been 

 described in which two inferior incisors are present. The single one ob- 

 served is very rarely found in adults, being a character more frequently 

 found in the young. It is in this species a remnant of a character else- 

 where constant, which does not disappear quite so soon as the teeth of the 

 whalebone whale, and superior incisors of the ruminant. But it is other- 

 wise with other species referred to Mastodon. No specimens of the Mas- 

 todontes angustidens, productus and longirostris, are recorded, in which 

 two inferior incisors are not present. For this reason the first and last- 

 named were placed by Grant and others in the genus Tetracaulodon. 

 Unfortunately this name was applied by its author to the M. americanus 

 only, a species which cannot enter the genus furnished with a pair of per- 

 sistent inferior incisors. It is also the type of Cuvier's Mastodon. % It 

 thus unavoidably becomes a synonym of the latter. 



There is no doubt that the presence of a pair of persistent inferior in- 

 cisors defines a genus as distinct from one in which there is not a pair of 

 permanent inferior incisors. I agree, therefore, with Grant and others, in 

 separating the Mastodons which present this character from the genus Mas- 

 todon, under another generic head. I believe, also, that the presence or 

 absence of a band of enamel on the superior incisors furnishes ground for 

 the recognition of distinct generic groups, and would be so used in any 

 other division of the Mammalia. It is often asked why it is necessary to 

 multiply generic names on such grounds. My answer is simply an ex- 

 pression of the law governing the case, based on the supposition that when 

 the species of animals and plants come to be fully known, the genetic 

 series will be found to be uninterrupted, excepting by the presence or. ab- 

 sence of characters which appear or disappear during the growth of a set 

 of individuals, which we on this account call a species, or refer to a genus. 

 The difference in the two cases consists in this : In the case of species, 

 the characters are numerous and are matters of proportion, size, color, 

 texture, etc., while in the case of the genus the character is single, and 

 marks one step in the serial chain of structural modifications. In the case 

 of the genus there is an actual addition or subtraction of some distinct 



* Palfeontological Memoirs of Falconer, i, p. 100, pi. 8. 



t Loc. cit., ii, p. 15. 



% Ossemeris Fossiles, ii, p. 252, Ed. 1884 : " Ann. Mus., 1806, viii, 272," teste Leidy. 



