T]![Tr.BKl;(TIA IX UNGULATES 67 



terms were carefully chosen with reference to the gradual rise of these 

 cusps from the single cone of the reptilian type, through the tritubercular 

 to the sexitubercular stages. 1 They have since been wholly or in part 

 adopted by Cope, Scott, Lydekker, Schlosser, 2 Flower, and lately by 

 liiitimeyer. 3 The tritubercular stem form has been recognized by 

 Doderlein and Fleischmann, but these authors have employed various 

 Greek symbols for the cusps. The latter has opposed the adoption of 

 similar terms for the main cusps of the upper and lower molars, upon the 

 ground that Cope and myself have mistaken the homologies : this 

 objection would be fatal to a uniform system of nomenclature for 

 the upper and lower cusps if it could be sustained, but a compre- 

 hensive survey of the Mesozoic trituberculates, especially of the 

 Amblotheriidse and Spalacotheriidse, leaves no doubt that the. anivm- 

 ciixji in fin /m/r/ 1 mo/a/-* n<l tin antero-internal cusp in fin 

 i' molars of the mammalia, are homologous -with fin- i-<j//i/i,i com an>/ 

 ,'ncli <>flit:r ; these cusps are invariably the most prominent, and are 

 always styliform in primitive types ; they always form the apices of the 

 primitive crown ; they persist in almost all mammals, while one or all of 

 the later cusps may disappear. 



This cardinal point established, it will be a great gain for palaeontology 

 and comparative odontology when the further truth is recognized that 

 fin jio^iliilitics of modification of ti/pc l/i tin* molars are limited, that 

 essentially similar types of teeth are evolved independently over and 

 over again, and that in course of what Schlosser has well termed 

 ' modernization ' we find such diverse orders as Primates, Ungulates, 

 Insectivores, Marsupials, Rodents, all exhibiting the same laws of dental 

 modification, and the same or similar ' secondary ' cusps, crests and 

 peripheral styles. 



Except in the Cetacea and Edentata, these modifications centre 

 around the simple tritubercular crown, which seems to possess unlimited 

 capacity of adaptation by the development of some parts and degeneration 

 of others, by changes of form and position, and by the addition of 

 secondary cusps. 



The first step is to distinguish and separate clearly the primary and 

 secondary regions of the primitive crown, for originally they have absolutely 

 different functions ; the part first developed in both upper and lower 

 molars is the anterior primitive triangle or tri</<m, which has a cutting or 

 piercing function ; out of its three cones all ' secodont ' types of molars 

 are evolved. The part next developed is the talon, or heel, which has 



1 See also " Evolution of Mammalian Molars to and from the Tritubercular Type," Am. 

 NaL, December, 1888. 



2 Schlosser, "Die Difl'erenzierinig des Siiugethiergebisses, " Biologisches Centralblatt, 

 Juni. 189H. 



3 Die Eociine Saugethier- Wtlt ron E</< i-L-in<i< , Zurich, 1891. 



